Democratic Rep. Jerry Costello’s announcement today that he won’t seek a 13th term in Illinois next year leaves Democrats with yet another competitive House seat to defend in 2012.

The now open 12th district leans Democratic, but Costello’s departure could force national Democrats to spend precious campaign cash on another House race in a challenging cycle.

His announcement came largely as a surprise to colleagues. Costello, the dean of the Illinois delegation, played a significant role in drawing the state’s aggressive new Congressional map.

His district did not change much in the redraw, but his absence nonetheless creates an opportunity for Republicans in southwest Illinois. Barack Obama won the area that is now the redrawn 12th district with almost 56 percent in the 2008 presidential election, but Sen. Mark Kirk (R-Ill.) carried it with just more than 54 percent two years later.

It didn’t take long for Democrats and Republicans to start suggesting potential successors to Costello.

Much of the speculation centers on former state Rep. Jay Hoffman (D), who announced a bid for the neighboring 13th district last month.

Democrats say Hoffman may instead run in the 12th district because it’s a more Democratic seat. He lives in the 12th district, plus he’s well-known from his previous campaigns in the St. Louis media market, which includes southwest Illinois.

Hoffman declined to address his political future in a statement responding to Costello’s retirement. He instead lauded the outgoing Member for his service.

“His retirement leaves some very big shoes to fill,” Hoffman said. “Now is not the time for speculation on any future political considerations, today is a day to honor Jerry Costello, a leader in our area.”

Democrats also identified state Rep. John Bradley as a potential candidate. His office did not return messages seeking comment today.

Some Democrats initially pointed to Costello’s son, newly appointed state Rep. Jerry Costello II, as a potential candidate. But the Congressman’s office confirmed that his son is not interested.

A couple of Republicans were already looking at challenging Costello. Former Belleville Mayor Roger Cook announced his candidacy over the weekend, and nurse Theresa Kormos is also running.

National Republicans courted Jason Plummer, the party’s nominee for lieutenant governor in 2010, to challenge Costello, and National Republican Congressional Committee Chairman Pete Sessions (Texas) took Plummer to a GOP conference meeting Sept. 21.

Despite the NRCC’s overtures, sources said Plummer had shied away from a House bid in recent weeks. But that may have changed now that Costello isn’t running. The Illinois Review, a conservative blog, reported that Plummer said today that he’s looking at running in either the 12th or 13th district. 

On the whole, Republicans were bullish about their prospects in the district.

“That’s good news for us,” Illinois Republican Party Chairman Pat Brady told Roll Call. “That’s going to be a big priority for us. I think that’s a big blow to our Democrat friends.”

But privately, other Republicans were more skeptical. One Illinois Republican described it as a “possibility, but it’s by no means easy.”

Even Democrats lamented that it’s an expensive district in which to run a campaign. The St. Louis media market is both pricey and inefficient because candidates inevitably buy advertisements that reach across the state line in Missouri.

It will be even more difficult for either candidate to break through the political noise of national campaigns. In addition, Sen. Claire McCaskill’s (D-Mo.) top-tier Senate race will dominate the St. Louis airwaves because of the media market overlap between the two states.

“It’s going to be very crowded and very expensive on a cost-per-vote basis,” one Illinois Democratic consultant said. “This is like running in New Jersey or Connecticut. No one in St. Louis covers politics on the other side of the river.”

Nonetheless, Costello skated by in his re-election campaigns for 12 full terms without spending too much cash. He reported $2.3 million in the bank at the end of June.

His retirement marks the fifth House Democrat to announce he is leaving Congress at the end of this term without seeking another office.