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 capture 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 gain:
—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.
Races to watch:
—Wisconsin: A last-minute infusion of campaign committee and outside money was the clearest sign of a suddenly close race between Republican Sen. Ron Johnson and the Democrat he ousted six years ago, former Sen. Russ Feingold.
—Indiana: An onslaught of stories about former Democratic Sen. Evan Bayh's residency and job search in his final year in office has undercut his candidacy. Republican Rep. Todd Young is determined to keep the seat in GOP hands. Sen. Dan Coats is retiring.
—New Hampshire: Republican Sen. Kelly Ayotte's rhetorical contortions in distancing herself from presidential nominee Donald Trump could prove costly as she faces a strong challenge from Democratic Gov. Maggie Hassan in this tightly contested state.
—Pennsylvania: Republican Sen. Pat Toomey, a frequent critic of President Barack Obama, launched an ad in the final days with words of praise for Toomey on gun control from — yes, Obama. Meantime, the GOP senator has declined repeatedly to say whether he would vote for Trump. The race with Democrat Katie McGinty is expected to go down to the wire in this 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.
Wait till December: Louisiana has 24 candidates on its top-two ballot, meaning a runoff is nearly certain on Dec. 10 to determine the next senator. Republican Sen. David Vitter lost his bid for governor and decided against another Senate run.