Washington (AFP) - Down-to-the-wire talks in Vienna this week will decide whether the United States can reach a landmark nuclear deal with Iran, but a fierce lobbying battle in Washington may decide if it survives.

For the last two years, those for and against an agreement to curb Iran's nuclear program have traded newspaper opinion pieces, rolled out dueling advocacy campaigns and lobbied "influencers" on the think tank circuit.

But, days before the June 30 negotiating deadline, the White House's political allies and its foes are significantly dialing up efforts to sell or sink the deal.

United Against Nuclear Iran -- led by former president George W. Bush's ambassador to the United Nations, Mark Wallace -- announced Tuesday it will plough vast resources into influencing the debate.

"We announced a national advocacy campaign with national television ads, print, multi-tiered social media and digital," Wallace told AFP.

"We have a multi-million-dollar budget and we are in it for the long haul. Money continues to pour in."

According to Trita Parsi, head of the pro-deal National Iranian American Council, "everyone is doing more. We are doing significantly more."

Both sides have in their sights members of Congress and the voters who might sway their decision.

Lawmakers will have a chance to vote on whatever deal comes from Vienna and opponents may manage to muster a majority against it.

But few believe Obama's critics, mostly Republicans, will have a big enough majority to make their verdict veto-proof.

Supporters of the deal point out that the history of Obama's domestic legislative victories shows there are perils even in triumph. In short, the vote count may matter as much as who wins.

Obama's landmark healthcare and financial reform both passed Congress by slender margins, prompting sustained efforts by opponents to re-legislate in Congress and re-litigate in the courts.

Republicans have tried to repeal "Obamacare" dozens of times and in a variety of guises -- the issue has even twice made its way into the Supreme Court.

Obama's opponents may have fewer avenues to scotch a deal with Iran, but even supporters admit multiple avenues do exist to slow or stall the agreement.

Congress will ultimately have to vote on repealing the those sanctions against Iran that were not introduced by White House order -- the bulk of US measures against Tehran.

- The vote -

"You need to make sure that public opinion is on your side," said Parsi. "At the end of this deal there is going to be a vote in favor or against this from Congress.

"If the vote was held today the President would survive it, but there is a long time left and the details of the final deal are not out yet."

"Clearly the other side feels like they have a chance still to defeated it, but more importantly, if the President wins with a very small margin, let's say he has 35 Senators supporting his veto and he only needs 34, that is 65 who are not in favor. With such a small margin you are going to give the opposition the incentive to try again, try again and try again. They only need to succeed once."

Polls have consistently shown public support for a proposed deal with Iran, but the picture in Congress ahead of 2016 elections is less clear and supporters are not resting on their laurels.

Parsi believes the most dangerous thing for the administration would be to believe the game is over. "It's not," he said.

In a battle that looks increasingly like the final stages of an election campaign, the opponents of a deal have picked up some significant endorsements.

Gary Samore, who for four years was Obama's principal advisor on arms control, believes the United States and its five negotiating allies should insist on tougher terms.

He has joined United Against Nuclear Iran in demanding unbridled international access to military facilities, interviews with Iranian nuclear scientists and other concessions that Tehran is highly unlikely to accept.

"I think P5+1 should insist that Iran meet our demands on the remaining issues to ensure an acceptable deal," he told AFP.

The "yes" camp is hoping that some Republican luminaries like former secretary of state Colin Powell or former national security advisor Brent Scowcroft, will ultimately lend their support.

But their biggest advocate, Obama, will come off the bench only if and when a deal is done, in an effort as big as anything his administration has done before.

"If a deal is reached you will see the most senior members of this administration speaking to it," said one of them.

Meanwhile, behind the scenes, the White House has been briefing Congress, journalists, foreign governments and non-governmental groups about the talks since a framework deal was reached in Switzerland in April.

"Our efforts to educate, inform and conduct outreach have been consistent and constant, ever since Lausanne, and of course before that we had a similar concerted push with our allies -- some of whom have voiced opposition to this -- and those who are on the fence, we are not taking any constituency for granted," the official said.