Rep. Joe Donnelly (D-Ind.) will announce Monday that he will run for Sen. Dick Lugar’s seat in 2012.

Butch Morgan, the head of Indiana’s 2nd Congressional District Democrats, notified supporters Saturday of Donnelly’s Senate candidacy in an email obtained by Roll Call.

“Congressman Joe Donnelly will be formally announcing Monday, May 9, 2011 that he will be a candidate for United States Senator from Indiana in 2012 and again for reelection in 2018!” Morgan wrote.

Donnelly, a three-term Democrat, was widely expected to jump into the Senate race after state lawmakers redrew his current House district so that it will include more Republicans in 2012.

He faces an uphill climb in Indiana, where Sen. Dan Coats (R) defeated then-Rep. Brad Ellsworth (D) by 20 points in last cycle’s Senate race. But although Republicans dominated the 2010 elections in the Hoosier State, Donnelly earned a slim victory over his GOP opponent in November.

Lugar, a six-term Republican, has easily won re-election in the past, but he is expected to receive a tough primary challenge from his right in 2012 from state Treasurer Richard Mourdock.