Factbox: Party Dominance In US Congress At Stake, Senate In Play

Washington: Power in the US Congress, a key issue for the next president, could shift on Tuesday in Senate and House of Representatives elections, with Democrats trying to break Republican majorities in both chambers and having a good shot in the Senate.

The presidential race between Democrat Hillary Clinton and Republican Donald Trump was expected to have a big impact.

Here are the basic on what's at stake and 10 key races.

US 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 a 54-seat majority, 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 Democrats have a 56-per cent chance of winning a Senate majority, said a New York Times poll on Monday.

The Republicans this year must defend 24 seats; the Democrats, 10. In 2018, Democrats must defend 23 seats, plus two independents' seats; Republicans, only eight.

US House, 435 Seats

Members of the House serve two-year terms and all members 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 with a majority of 247 seats over the Democrats' 188.

The Republican leader is Speaker of the House Paul Ryan of Wisconsin; the Democrats' leader is Nancy Pelosi of California.

To win a majority, Democrats need to gain 30 seats. Polls project Democratic gains of only five to 20 seats.

Key Senate races:

Arizona. Republican Senator John McCain, 80, faces an unexpectedly strong challenge from Democrat Ann Kirkpatrick, 66. McCain, a war hero who was the presidential nominee in 2008, has had some very public differences with Trump.

Florida. Republican Marco Rubio, the failed presidential contender, faces Democratic Representative Patrick Murphy, 33.

Rubio, 45, had been expected to end his political career after losing Florida's Republican presidential primary to Trump, but he changed his mind and ran for a second Senate term.

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 be a lobbyist. Bayh or Young will replace Republican Senator Dan Coats, who is retiring.

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. Duckworth, 48, is a double-amputee Iraq War veteran.

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.

New Hampshire. Democratic New Hampshire Governor Maggie Hassan, 58, is trying to oust Senator Kelly Ayotte, a 48-year-old Republican seeking a second term.

North Carolina. Republican Senator Richard Burr, 60, is seeking re-election against Democrat Deborah Ross, 53.

Ohio. Republican Rob Portman, 60, is working to see off Democratic challenger Ted Strickland, 75. Portman initially endorsed Trump, but later withdrew that backing and avoided appearing with Trump or even mentioning his name.

Pennsylvania. Republican Senator Patrick Toomey, 54, faces Democrat Katie McGinty, 53, in the most expensive US Senate contest in the country. Toomey has refused to take a position on Trump. McGinty has criticized him for this.

Wisconsin. Democrat Russ Feingold, 63, is trying to unseat Republican Ron Johnson, 61. Johnson unseated Feingold in 2010.