By Richard Cowan and David Morgan
WASHINGTON (Reuters) - The Democrats were slightly favored to wrest control of the U.S. Senate from the Republicans in Tuesday's elections, according to some analysts, with the outcome helping to determine how difficult it will be for the next president to pass legislation.
Democrats' hopes for big gains in the Senate and the House of Representatives, both now controlled by Republicans, were tempered in the closing days of the 2016 campaign, even if Democratic nominee Hillary Clinton wins the White House.
Two weeks ago, Democrats hoped to sharply reduce the Republicans' 246-seat House majority and grab control of the Senate. But the FBI may have dashed those ambitions by reigniting a controversy about Clinton's emails while she was secretary of state, congressional aides and analysts said.
Americans are voting to choose either Clinton, also a former U.S. senator, or Republican Donald Trump, a businessman who has never previously run for political office, and to fill 34 of the 100 Senate seats and all 435 House seats.
Senate results may not be available for some time because of the number of close races, analysts said.
Polling website RealClearPolitics.com on Tuesday showed Democrats likely to capture one Senate seat now held by Republicans and listed eight other Republican seats as toss-ups. House races showed no clear trend.
Projections from the New York Times and forecasting website FiveThirtyEight.com showed Democrats with a just over 50-percent likelihood of having Senate control when it convenes on Jan. 3.
An analysis of Senate races by political scientist Larry Sabato's "Crystal Ball" project at the University of Virginia projected the election would end with Democrats and Republicans each holding 50 seats.
Continued Republican dominance in Congress could stymie any legislative agenda pursued by Clinton. A Trump victory, coupled with a Republican Congress, could spell a swift demise for Democratic President Barack Obama's health reforms.
The emails controversy erupted again after Federal Bureau of Investigation Director James Comey said on Oct. 28 his agency would examine newly discovered emails. A senior Democratic aide said it could reduce to as few as 12 the number of House seats Democrats may capture from Republicans, well short of the 30 they need to take control of the chamber.
Comey told Congress on Sunday that after the latest review, he was standing by his decision in July that no criminal charges were warranted against Clinton.
But he made the statement only two days before the election and may have been too late to overcome the damage done to Democrats aiming to regain control of the House for the first time since 2010 and the Senate for the first time since 2014.
Still, House Democratic caucus chairman Xavier Becerra held out hope for a more than 20-seat gain in light of high early voter turnout, especially among Latinos. "We'll at least be able to step up pressure on a thinned-out Republican majority," he told Reuters.
To win control of the Senate, Democrats would have to score a net gain of five seats. Republicans hold 54 Senate seats to 44 Democratic seats and two independents who align themselves with Democrats.
Even a Republican majority could be divided against itself, with party members disagreeing on issues such as whether to give a Supreme Court nominee a confirmation vote.
New Hampshire Senator Kelly Ayotte, a Republican in a close reelection fight, appealed to independent voters on Tuesday with a video saying: "It's going to take someone who can stand up to both parties when they're taking us in the wrong direction."
(Reporting by Richard Cowan, Susan Cornwell and David Morgan; Editing by Kevin Drawbaugh and Grant McCool)