Romney and Santorum (Jae C. Hong/AP)

Mitt Romney fought for frontrunner status in front of a live national audience Wednesday night in the last debate before the crucial primaries on Feb. 28 in Michigan and Arizona.

In his opening statement at the CNN GOP debate, Romney said "I want to restore America's promise, and I'm going to do that --" prompting immediate applause from the audience in the Mesa Arts Center in Arizona. Instead of continuing, he stopped mid-sentence, adding: "That's good enough. As George Costanza would say, 'when they're applauding, stop.' Right?"

The former Massachusetts governor, who was born and raised in Michigan where his father served as governor, challenged former Pennsylvania Sen. Rick Santorum in his first answer of the evening, stating that Santorum voted in Congress to fund Planned Parenthood, the Department of Education, voted to raise the debt ceiling "five times," and to maintain the Davis-Bacon Act, which requires prevailing wages to be paid for public works projects. "Senator, during your term in Congress, the years you've been there, the government's doubled in size," Romney said.

Santorum defended his record, especially on spending, noting his positive rankings from the American Conservative Union.

The back-and-forth between Santorum and Romney was a public display of the battle that has been playing out in campaign ads and stump speeches issued by each candidate in the preceding weeks.

In one heated exchange, Santorum charged that Romney's healthcare plan in Massachusetts was a model for Obamacare. Romney hit back at Santorum saying "the reason we have Obamacare is... that you supported the pro-choice Senator of Pennsylvania and he voted for Obamacare." Santorum endorsed Arlen Specter for U.S. Senate in Pennsylvania in 2004 against a more conservative Republican challenger-- Rep. Patrick J. Toomey. Specter in 2009 announced his decision to run as a Democrat in the 2010 election.

The 2012 presidential race hinges on what happens in Michigan on Tuesday, so of course all of the Republican candidates are in Arizona. It's the 20th debate of the campaign, and the last--presumably--before Super Tuesday on March 6.

During the debate, read and contribute to our liveblog, which will feature real-time discussion and analysis from Yahoo News and ABC News journalists, as well as tweets from Yahoo News reporters on the ground in Arizona. (You can follow the liveblog below, or go to this page, which has also been optimized for tablet and mobile devices.)

The liveblog will feature Rick Klein, the senior Washington editor for ABC News' "World News with Diane Sawyer"; Olivier Knox, the White House correspondent for Yahoo News; Walter Shapiro, who writes the "Character Sketch" column for Yahoo News; Chris Suellentrop, the deputy editor for blogs at Yahoo News, including The Ticket; and Z. Byron Wolf, the political editor of ABCNews.com and a deputy political director for ABC News.

Also participating will be Jeff Greenfield, Joshua Green, and Weston Kosova.

Jeff Greenfield, a veteran political correspondent and the host of PBS' "Need to Know," is the author, most recently, of Then Everything Changed, which will be published in paperback next month.

Joshua Green is a national correspondent for Bloomberg Businessweek. Follow him on Twitter at @JoshuaGreen.

Weston Kosova is the Washington editor for Bloomberg Businessweek.

Firing back at Republican critics, the White House said Wednesday that President Obama is doing everything he can to deflate rising gasoline prices that threaten the fragile economic recovery.

Spokesman Jay Carney dryly dismissed the increasingly sharp attacks from Republican presidential candidates as "random statements by politicians seeking office" and mocked their proposals to ease the pain at the pump as relying on "magic solutions," "magic beans," or a "magic wand."

Obama "fully appreciates the impact of higher gas prices on average Americans trying to make ends meet," Carney said. "He's very aware of the impact that it has and fully understands the anxiety it creates."

Security and staff stand by a shadow of Rick Santorum at the Sabbar Shrine Center in Tucson, Arizona. (Eric Ga …

TUCSON, Arizona --The Tucson Tea Party didn't take any chances with security when they hosted Republican presidential candidate Rick Santorum at a meeting here Wednesday, just a few miles from where Rep. Gabrielle Giffords was shot last year while meeting with constituents.

"Sen. Santorum is not intending to speak outside, for security reasons," the Tucson Tea Party website announced in advance of the event, which was held at the Sabbar Shrine Center.

Lining up for the event, supporters passed hand-written signs declaring  "NO WEAPONS" and "NO LIQUIDS." Volunteers wearing red t-shirts emblazoned with the local tea party logo stopped each person for a bag check, while beefy private security agents roamed in and out of the building. Police were on hand to direct traffic.

Well before Santorum arrived, Jeffrey Prather—who founded the Tucson-based personal defense training agency Warriorschool—cased the building, taking pictures from several angles in search of possible possible security breaches. Warriorschool led security operations at the center.

"We'd rather be overly cautious with a presidential candidate," Tucson Tea Party lead organizer Ralph Kayser said.

Mitt Romney in Chandler, Arizona (Justin Sullivan/Getty Images)

CHANDLER, Ariz.—The campaign signs at Mitt Romney's first rally in Arizona in more than a week looked like a last minute print job. Red and white with a far more sparse design than his regular 2012 logo, the signs read "Mitt Romney AZ" and were printed on heavy letter stock paper that looked more like fancy resume paper than political signage.

As Romney and his wife, Ann, took the stage at a tiny Christian school here just outside Phoenix, supporters enthusiastically waved their "AZ" signs in the air—and the signs, unlike the regular sturdier Romney 2012 placards, rippled precariously as though they might tear at any second.

The signs gave an air to what has generally been a last minute feel to the Arizona primary campaign as it heads into its final days. Unlike previous voting states, the television airwaves here aren't being blanketed by campaign ads, and few, if any, political signs adorn the streets. There's a good reason why: Two weeks ago, the state looked like a sure thing for Romney, as he was up by more than 25 points over Rick Santorum, his closest rival. The wide lead enabled Romney's campaign to start shifting its energies to Michigan.

But, the race has tightened considerably in recent days, prompting some alarm among Romney supporters here. It's unclear if Romney is going to increase his presence in Arizona in the coming days. While his campaign had been expected to focus largely on Michigan ahead of next Tuesday's primary, an aide acknowledged the schedule remains up in the air.

On Tuesday, a CNN/Time Magazine poll found Romney and Santorum statistically tied in the state, 36 percent to 32 percent. But that was contradicted by a NBC News/Marist poll released Wednesday, which found Romney leading Santorum by 16 points, 43 percent to 27 percent.

Asked Wednesday which poll he believed more, Arizona Rep. Jeff Flake, one of Romney's most high profile supporters here, acknowledged the race has gotten much tighter in recent days. "I think it's somewhere in the middle," Flake said, when asked if he believes Romney has a 16 point lead or 4 point lead. "But what counts is that Romney is going to fight up until Election Day and win."

Trailing by a double-digit margin in the polls in the waning weeks of his 2006 Senate reelection campaign in Pennsylvania, Rick Santorum went nuclear. He did so not by launching vicious new attacks on his soon-to-be-victorious Democratic opponent, Bob Casey, but by indulging his own obsession with mushroom-cloud politics. He began speaking incessantly about the potential threat from a nuclear-armed Iran.

This was not a clever consultant's ploy to give the two-term Pennsylvania senator an aura of foreign-policy gravitas. Rather it was Santorum's laudable decision--knowing he was politically doomed--to go down fighting with an issue that aroused his passions. Santorum, who can be guilty himself of Newt Gingrich-style grandiosity, modeled himself after Winston Churchill and his thunderous warnings about Nazi Germany from the parliamentary backbenches in the 1930s. Santorum labeled his oft-delivered speech on the Iranian menace as "The Gathering Storm," taking its title from the first volume of Churchill's postwar memoirs.

Santorum did not stint with his 1930s jack-booted metaphors as he linked the Iranian president, Mahmoud Ahmadinejad, to the usual suspects. "Ahmadinejad, like Hitler and Mussolini, intends to conquer the world," Santorum said in an apocalyptic tone that suggested that Iran (population: 70 million) was size of China. "This is not a hidden agenda. His goal is to establish a caliphate. Like Khrushchev, he wants a nuclear arsenal, and he is building the same sort of frightening global alliances that enabled the Soviet Union to put missiles near us."

These days, Santorum is under fire for his comments in 2008 that Satan stalks America. But Santorum's 2006 speech about Iran (cue the 1979 clip of the Ayatollah Khomeini excoriating America as "the Great Satan") is far more relevant to the Oval Office. The challenge for any president is how to interpret ambiguous national-security information, whether it is about Iraq's purported weapons of mass destruction or the status of the Iranian nuclear program. Santorum's fire-bell warnings about Iran strongly suggest that he is a disciple of the Dick Cheney better-wrong-too-early-than-right-too-late school of aggressive intervention.

This is not to say that Mitt Romney and Newt Gingrich raise doves as a hobby. In Romney's major foreign policy address last October, he unequivocally if vaguely declared, "Iran obtaining a nuclear weapon is unacceptable." Campaigning in Ohio earlier this month, Gingrich warned, "You think about an Iranian nuclear weapon. You think about the dangers to Cleveland, or to Columbus, or to Cincinnati, or to New York." The former House speaker did not explain how or why Iran, which has not yet developed an operational nuclear weapon, intends to target Cincinnati.

Ron Paul is the uncompromising Republican naysayer about a potential war over Iran's nuclear ambitions. "The greatest danger is overreacting," Paul said during a December debate. "That's how we got into that useless war in Iraq." There is a constituency in American politics for this kind of skepticism about preventive war. Unfortunately for Paul, it isn't in the Republican Party. Three-quarters (74 percent) of all GOP voters are willing to support as a last resort American military action to prevent Iran from obtaining nuclear weapons, according to a Pew Research Center poll, conducted earlier this month. And three-fifths (62 percent) of these Republicans would cheer an Israel assault on Iranian nuclear facilities.

President Obama with Vice President Biden (Susan Walsh/AP)President Obama and Mitt Romney released dueling tax plans Wednesday, and helped define some key differences between the two candidates in this election year.

Obama 's proposal would overhaul the corporate tax code with the goal of encouraging job creation and investment in the U.S., and reducing tax incentives for shifting operations overseas.

"We want to restore a system in which American businesses succeed or fail based on the products they make and the services they provide," Treasury Secretary Tim Geithner told reporters in a briefing Wednesday morning, "not on the creativity of their tax engineers, or the lobbyists they hire."

The proposal offered by Romney -- who, despite some recent stumbles, is still seen as the likeliest Republican nominee -- is even more sweeping: It cuts and revamps both individual and corporate taxes, and calls for shrinking the size of government.

The Obama administration's plan, which requires congressional approval, would cut the corporate tax rate from 35 percent to 28 percent, with manufacturers receiving preferences that would allow them pay an effective rate of 25 percent, Geithner said. (The White House recently launched a push to encourage domestic manufacturing.)

The plan also would use the tax code to promote research and development, and clean energy. But it would not address the individual income tax -- an area of the tax code that Republicans in particular have stressed the need to refashion.

The White House had a classic Washington explanation for why President Barack Obama won't be watching Wednesday's Republican presidential debate: He wants to spend more time with his family.

"I suspect, knowing him, knowing his viewing habits, that he will not watch it. He has a family at home," Jay Carney, the White House press secretary, told reporters at his daily briefing.

"He tends to, when he watches TV at all, it's either sports or a movie, so I don't expect he will. But the president obviously keeps up with what's in the news. And will I'm sure be aware of the general back and forth of the debate come tomorrow morning," the spokesman said.

Asked whether Obama might watch the debate for clues as to his potential challenger's habits, tells, strengths and weaknesses, Carney verbally shrugged. "There is ample time between now and early November for him to prepare for what will be debates with his opponent, once that opponent emerges from this process," he said.