Most competitive U.S. Senate races

The political math in states where tight races could determine control of the Senate:

North Carolina

Gay rights battle + incumbent slow to respond to challenger = tougher race for Republican.

Sen. Richard Burr, a Republican, was thought to be a safe bet for re-election against Deborah Ross, his Democratic challenger, a former state representative and head of the state's American Civil Liberties Union chapter. But North Carolina has been roiled by a fight over bathroom rights for transgender people. The national Republican Party is trying to save Burr's bid.

Pennsylvania

Suburban women + Clinton strength + minority votes = Democratic edge.

Katie McGinty, a Democrat, is trying to become Pennsylvania's first female senator with a bid to unseat Sen. Pat Toomey, a Republican who has managed to keep a moderate image around the state because of a failed gun safety bill that bore his name. Hillary Clinton's coattails — and Donald Trump's anvil — have made a difference here.

Debate misstep on Trump + strong challenger = imperiled GOP incumbent.

Sen. Kelly Ayotte, a Republican, is suffering from an uncomfortable association with Trump, most notably when she had to release a statement walking back her suggestion during a debate that Trump was a role model for children. Gov. Maggie Hassan, her Democratic challenger, is getting major assistance from the national party.

Missouri

War vet + breakthrough ad + anti-establishment sentiment = possible upset.

Sen. Roy Blunt was on no lists of endangered incumbents. Then his Democratic challenger, Jason Kander, an Afghanistan war veteran who is Missouri's secretary of state, conjured up an ad in which he assembled an assault rifle blindfolded while speaking of gun rights. Blunt's decades of service in Washington are also out of style this year.

Revolving door + Trumplandia = it's tough to come home again.

Evan Bayh, a Democrat, is trying to reclaim his Senate seat but is finding that his time as a D.C. insider is a liability. Rep. Todd Young, with the help of outside money, is painting a dark picture of Bayh as a greedy lobbyist.