Factbox: Tight races put U.S. Senate in play as voters pick new Congress

(Reuters) - Majority control of the U.S. Senate was up for grabs in Tuesday's election, with a handful of extremely tight races seen as too close to call even as the polls began closing. Republicans were fighting to defend majorities in the Senate and the House of Representatives. Democrats hoped to capture Senate control, but were less optimistic about overcoming the Republicans' larger advantage in the House. The outcomes in both chambers will help determine how hard it will be for either Democrat Hillary Clinton or Republican Donald Trump to get things done as president. The following are facts on the stakes and races to watch: U.S. Senate, 100 seats. Senators serve six-year terms. A third of the Senate is up for re-election every two years. Procedural rules in the Senate mean 60 votes are needed to advance major initiatives. Republicans entered the election with 54 seats, led by Kentucky's Mitch McConnell, versus the Democrats' 44 seats and two independent seats. The Democrats' leader in the next Senate is expected to be New York's Chuck Schumer. The Republicans this year were defending 24 seats; the Democrats, 10. In 2018, Democrats must defend 23 seats, plus two independents' seats; Republicans, only eight. U.S. House, 435 seats Members of the House serve two-year terms and all are up for re-election every two years. To advance most bills in the House, 218 votes or more are needed. Republicans went into the elections holding 246 seats to the Democrats' 186. There were three vacancies. The Republican leader is Speaker Paul Ryan of Wisconsin; the Democrats' leader is Nancy Pelosi of California. To win a majority, Democrats need to gain 30 seats, but congressional aides and analysts were increasingly skeptical on Tuesday that the party would achieve gains that large. Senate races: Arizona - Republican Senator John McCain, 80, faces an unexpectedly strong challenge from Democratic U.S. Representative Ann Kirkpatrick, 66. McCain, a Vietnam War hero who was the presidential nominee in 2008, has had some public differences with Trump. Florida - Republican Marco Rubio, the failed presidential contender, faces Democratic Representative Patrick Murphy, 33. Rubio, 45, had been expected to enter the private sector after losing Florida's Republican presidential nominating contest to Trump, but he changed his mind after an active recruitment effort by party leaders. A Quinnipiac University poll late on Monday showed Rubio leading Murphy by 50 percent to 43 percent. Indiana - Democrat Evan Bayh, 60, is trying to recapture his Senate seat, facing Republican Representative Todd Young, 44. Bayh left the Senate in 2011 to work for a lobbying firm. Bayh or Young will replace Republican Senator Dan Coats, who is retiring. Political website RealClearPolitics on Tuesday had Young building on a small lead over Bayh in two polls since Oct. 27. Illinois - Republican Senator Mark Kirk is trying to fend off a challenge from Democratic Representative Tammy Duckworth. Kirk, 57, suffered a stroke that sidelined him for much of 2012 and is considered one of the most vulnerable Republicans seeking re-election this year. Duckworth, 48, is a double-amputee Iraq War veteran. Louisiana - Republican Senator David Vitter is retiring, opening the door to a crowd of would-be successors. Two dozen candidates from both parties are on Tuesday's ballot. If no candidate gets at least 50 percent of the vote, a runoff election between the top two will be held on Dec. 10. Missouri - Republican Senator Roy Blunt, 66, faces a challenge from Democrat Jason Kander, 35, a veteran of the Afghanistan war who is now Missouri's secretary of state. Blunt and Kander were separated by 2 percentage points or less in polls between Oct. 24 and Nov. 5, RealClearPolitics said. Nevada - Republican Representative Joe Heck, 55, and Catherine Cortez Masto, 52, a former Democratic state attorney general, are fighting to replace retiring Senator Harry Reid, who has worked hard for Masto. She had a narrow lead in recent polls. New Hampshire - Democratic Governor Maggie Hassan, 58, is trying to oust Senator Kelly Ayotte, a 48-year-old Republican seeking a second term. Ayotte had a 1.5 percentage point lead over Hassan in polls between Oct. 28 and Nov. 6, RealClearPolitics said. North Carolina - Republican Senator Richard Burr, 60, is seeking re-election against Democrat Deborah Ross, 53, a former state legislator. Polling on Monday by Quinnipiac showed the two locked in a dead heat. Ohio - Republican Rob Portman, 60, is working to see off Democratic challenger Ted Strickland, 75, a former governor. Portman initially endorsed Trump, but later withdrew that. Portman was leading by 56 percent to 38 percent in a Nov. 3 Quinnipiac poll. Pennsylvania - Republican Senator Patrick Toomey, 54, faces Democrat Katie McGinty, 53, in the most expensive U.S. Senate contest in the country. Toomey has refused to take a position on Trump. An Oct. 30-Nov. 4 poll by the Allentown Morning Call and Muhlenberg College had Toomey with a small lead, 43 percent to 42 percent. Wisconsin - Democrat Russ Feingold, 63, is trying to oust Republican Ron Johnson, 61. Johnson unseated Feingold in 2010. The two were separated by only a percentage point in a late October Marquette University poll. (Reporting by Susan Cornwell, Andy Sullivan and Richard Cowan; Editing by Kevin Drawbaugh and Peter Cooney)