Updated 11:37 a.m. | Rep. Dan Burton (R-Ind.) will retire this year after serving 15 terms in the House, according to Hoosier State sources.

Burton will announce his retirement on the state House floor, per state Sen. Mike Delph (R), a former Burton aide and one of the Congressman’s closest political allies in Indianapolis.

Burton’s departure will leave his redrawn central Indiana district open. The 5th district heavily favors the GOP, and several Republicans had already lined up to challenge Burton in the  primary, including former Rep. David McIntosh, former U.S. attorney Susan Brooks, former Marion County Coroner John McGoff and attorney Jack Lugar.

In 2010, Burton garnered just 30 percent of the vote in the seven-way primary. For that reason and others — including changes to his district — Burton earned a spot on Roll Call’s Top 10 Most Vulnerable List in November.

Burton’s retirement marks the seventh House Republican to leave Congress at the end of this year without seeking another office, according to Roll Call’s Casualty List. Eleven House Democrats have also chosen to leave and not seek other office.

A Democrat, state Rep. Scott Reske, had announced his bid for the 5th district seat. But Republicans are expected to hold the seat in 2012.

Burton’s retirement news was first tweeted by the Indianapolis Star’s Mary Beth Schneider.

Update 11:37 a.m. 

State Rep. Woody Burton (R), the Congressman’s brother, confirmed the retirement news to Roll Call. Woody Burton also said he will not run for his brother’s seat, but sources said Delph is considering it.

A former district director for Burton, Delph was one of the first to know about Burton’s retirement —  an announcement that came as a surprise to many in Indiana and on Capitol Hill. Even Burton’s Washington, D.C., staff did not know about the Congressman’s retirement until they read about it on Twitter.

Through his spokeswoman, Delph declined to comment today on his future political plans. But his political consultant, Chris Faulkner, confirmed he’s thinking about the race.

“I know that Delph is taking a look at the situation closely and assessing what’s best for him and his family,” Faulkner said. “I’d be hard-pressed to see Burton supporting anyone else in that primary other than Mike Delph.”

If Delph or anyone else wants to jump into the already crowded primary, they’ll have to do it quickly. The filing deadline is Feb. 10 for the May 8 primary.