Illinois Rep. Bill Foster announced Friday he will not run for Senate next year and instead endorsed fellow House Democrat Tammy Duckworth, who joined the race last month.
That decision by Foster, who was actively mulling a Senate bid, brings Democrats one step closer to avoiding a potentially damaging primary in what may be the party's best opportunity to pick up a seat.
"I have decided to avoid what would be a costly and, ultimately unnecessary, primary," Foster said in a statement.
"This decision was a difficult one," he added, "but it was made much easier by the fact that we have such a terrific candidate in Tammy Duckworth."
Illinois Democrats, including Sen. Richard J. Durbin, said they were actively working to avoid a primary. Foster was one of four Democrats considering a challenge to Kirk, but he now joins Rep. Cheri Bustos in passing on the race.
Still contemplating a run is Rep. Robin Kelly, who hails from the Chicago-based 2nd District. As of Duckworth's March 30 announcement, Kelly was "still considering the race," according to an aide.
Democrats must net five seats to ensure Senate control. Kirk is a top target, as he is a Republican running in a state President Barack Obama carried with 58 percent in 2012.
The general-election race is rated a Tossup by the Rothenberg & Gonzales Political Report/Roll Call.