U.S Sen. Mark Kirk will face a challenge from Congresswoman Tammy Duckworth, who announced Monday she'll run for the seat. It's unknown who else will vie for the spot, but it's already expected to be a tight race.
Duckworth, who was elected to the U.S. House in 2012, took to YouTube to declare her candidacy.
"I'm running for the U.S. Senate in 2016 because it's time for Washington to be held accountable, and to put Illinois' families and communities first," she said in the video.