Control of the Senate is on the ballot in November, with Republicans fighting to hold their majority while defending far more seats in Democratic-leaning states. A look at the Senate landscape:
Current party breakdown: 54 Republicans, 44 Democrats, two independents who caucus with the Democrats.
Seats on the ballot: 24 Republican-held seats, 10 Democratic-held seats.
To hold the majority, Democrats need a net gain of four seats if Hillary Clinton wins the presidency, as the vice president would break ties; five seats if Republican Donald Trump is elected president. The last time the Senate was tied 50-50 was in the first year of President George W. Bush's presidency with Vice President Dick Cheney breaking any ties.
Likely Democratic gains:
—Illinois: Sen. Mark Kirk, one of the more moderate Republicans, recovered from a 2012 stroke and returned to the Senate. He faces Democratic Rep. Tammy Duckworth, who lost both legs and partial use of an arm in a rocket-propelled grenade attack in Iraq. Illinois favors Democrats, especially in a presidential election year.
—Wisconsin: Republican Sen. Ron Johnson is in a close race against the Democrat he ousted six years ago, former Sen. Russ Feingold. But Johnson is fighting against Wisconsin's Democratic tilt in a presidential election year.
Races to watch:
—Indiana: Former Democratic Sen. Evan Bayh is trying to get his old seat back. Republican Rep. Todd Young is determined to keep the seat in GOP hands. Republican Sen. Dan Coats is retiring.
—New Hampshire: Republican Sen. Kelly Ayotte faces a strong challenge from Democratic Gov. Maggie Hassan in this tightly contested state.
—Pennsylvania: The race between Republican Sen. Pat Toomey and Democrat Katie McGinty is close in a pivotal state.
—Missouri: Republican Sen. Roy Blunt, a longtime member of Congress, is in one of the toughest races against Democrat Jason Kander, Missouri's secretary of state and a promising young recruit for the Democrats.
—North Carolina: Republican Sen. Richard Burr faces energized Democrats; former American Civil Liberties Union lawyer Deborah Ross hopes to score an upset.
—Nevada: This is the one seat Republicans have a chance of flipping as GOP Rep. Joe Heck is in a tight race with Democrat Catherine Cortez Masto, Nevada's former attorney general. The Senate Democratic leader, Harry Reid, is retiring after five terms.
—Florida: GOP Sen. Marco Rubio is in good shape due to a solid campaign and Democrats' reluctance to invest heavily in the expensive state to back Rep. Patrick Murphy. But Rubio could still be in trouble depending on Trump's performance.
Likely to stay in GOP hands:
—Ohio: Republican Sen. Rob Portman has run a strong campaign, defining former Democratic Gov. Ted Strickland early on. Portman is widely expected to prevail regardless of Trump's performance.
—Arizona: Five-term GOP Sen. John McCain is facing a determined challenge from Democratic Rep. Ann Kirkpatrick, but has maintained a lead in polls.
Wait till December: Louisiana has 24 candidates on its top-two ballot, meaning a runoff is widely expected on Dec. 10 to determine the next senator. Republican Sen. David Vitter lost his bid for governor and bowed out.