Rep. Jerry Costello (D-Ill.) will not run for reelection, according to a spokesman, which could give Illinois Republicans a chance to pick off his downstate Illinois district.

In a statement Tuesday, the 11-term lawmaker made his decision official.

"It has been a privilege and an honor to serve in the U.S. Congress for the past 23 years. However, I said when I was elected in 1988 and many times since that I did not intend to stay in Congress forever as I had other interests that I wanted to pursue," he said.

Costello has comfortably held on to his slightly Democratic-leaning district for decades. He also played a big part in Illinois Democrats' decision to destabilize neighboring Rep. Tim Johnson (R-Ill.) when they redrew the state's congressional map rather than make his district safer.

That move may come back to bite them, as Costello's new district has swaths of rural, Republican-leaning territory and is only slightly Democratic-leaning.

"They could have done more to shore it up," said David Wasserman, the House editor at the Cook Political Report. "Now they risk losing both the Johnson district and his district."

The new district would have backed both Sen. Mark Kirk (R-Ill.) and GOP gubernatorial nominee Bill Brady in 2010, although a Democratic operative pointed out that Sen. John Kerry (D-Mass.) got 51.3 percent of the vote there, making it more Democratic than all but eight seats House Republicans currently hold.

The National Republican Congressional Committee has already been on the air running TV and radio ads against Costello. Former Belleville Mayor Rodger Cook announced his campaign last week, and Jason Plummer, the GOP's 2010 nominee for lieutenant governor, may also be considering the race.

Costello's retirement was first reported by KMOX in St. Louis.

— This post was updated at 12:15 p.m.