Rep. Jerry Costello (D-Ill.) will not seek reelection in 2012, freeing up a district that Republicans may have a chance to win.

Costello’s office confirms that the 12-term congressman has decided not to seek a 13th full term.

“It has been a privilege and an honor to serve in the U.S. Congress for the past 23 years,” Costello said in a statement. “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.”

Despite representing a East St. Louis-based district that leans only slightly Democratic, Costello has held on with ease for two decades, including taking 60 percent of the vote in a very tough year for Democrats in 2010.

But his district went for Sen. John Kerry (D-Mass.) with just 52 percent of the vote in the 2004 presidential race and for then-Sen. Barack Obama with 56 percent in 2008 — a relatively small margin given Obama’s strength in his home state.

And actually, the district is slightly friendlier for the GOP under the new congressional map drawn by Illinois Democrats.

The new map could unseat upwards of four or five incumbent Republicans in the state, but by weakening those GOP-held districts, Democrats actually made Costello’s district slightly less Democratic — about a 55 percent Obama district. It is now one of the more competitive districts in the state heading into 2012.

The district also went for the GOP’s Senate and governor candidates in 2010.

Democrats point out that, even after the GOP’s big gains in 2010, they hold fewer than a dozen districts that gave Kerry more of the vote than Costello’s district — suggesting that district is still an uphill battle for the GOP.

Former Belleville Mayor Roger Cook (R) announced his campaign for the seat this week.

According to the Cook Political Report, Costello is the 13th House Democrat who won’t seek reelection, but only the fifth who isn’t seeking higher office.

Six House Republicans are seeking higher office; none are retiring outright.