Republican Terri Lynn Land and Democratic U.S. Rep. Gary Peters are vying to fill the seat being vacated by retiring Sen. Carl Levin.
As longtime Sen. Tom Harkin retires, Republican State Sen. Joni Ernst and Democratic Congressman Bruce Braley are vying for the seat.
Republican candidate Mike Round's is losing his safe margins in the race for South Dakota's senate seat.
Senate incumbent Lamar Alexander faces lawyer and Democrat Gordon Ball.
In Wyoming, incumbent Republican Senator Mike Enzi is running for a fourth term.
An incumbent Democrat faces a Republican challenger in a special election to fill the seat of former Hawaii senator Daniel Inouye, who died in 2012.
Democratic senator Mary Landrieu is fighting to keep her job against Republican challenger, Bill Cassidy.
In the latest installment of our Senate Tracker series, we turn to the race in Kansas.
Incumbent Senator Mark Begich is facing a challenge from Dan Sullivan, a former state attorney general under then-Gov. Sarah Palin.
In Colorado, Democratic U.S. Senator Mark Udall is facing a challenge from Congressman Cory Gardner.
Democrat Alison Lundergan Grimes, Kentucky's secretary of state, is trying to unseat longtime U.S. Senator Mitch McConnell.
Incumbent Democrat Mark Pryor and Republican Congressman Tom Cotton are battling for a U.S. Senate seat in Arkansas.