Rep. Bill Foster (D-Ill.) announced Friday that he would not run for the Senate in 2016, throwing his support behind the early Democratic frontrunner, Rep. Tammy Duckworth, in a move that spares her at least one well-funded primary challenger.

Foster’s announcement is the latest bit of good news for Duckworth and the Democratic establishment. Many Democratic strategists view her as the strongest possible challenger against Sen. Mark Kirk (R). Foster, who is independently wealthy, could have instantly infused his campaign with millions of dollars, complicating Duckworth's path to the nomination.

“This decision was a difficult one but it was made much easier by the fact that we have such a terrific candidate in Tammy Duckworth,” said Foster in a statement. He said that the “resources on the Democratic side should be focused on winning that seat back for the people of Illinois, not on Democrats fighting one another.”

Duckworth, an Iraq War veteran who was awarded a Purple Heart after losing her legs during a 2004 mission, announced her campaign last month. She is the only major Democratic candidate in the race.

Illinois is seen as a key battleground in the fight for the Senate majority. Democrats believe they have a good chance at defeating Kirk, since Democratic turnout is expected to be high in Illinois because of the presidential race. Illinois tilts heavily Democratic in presidential years.

Kirk, who returned to the Senate in 2013 after suffering a stroke, has been actively preparing for his reelection bid.

One potential Democratic candidate to watch is Rep. Robin Kelly. She represents a district based in the South Side of Chicago.

Kelly, who is African American, told The Washington Post in February: "I feel like I have a good resume that I bring to the table. I have a pathway to victory."