Rep. Mark Kirk (R-Ill.) began telling supporters on Wednesday that he will run for Senate in 2010, according to knowledgeable GOP sources. Kirk’s move follows the news that Illinois Attorney General Lisa Madigan (D) has decided to run for re-election and will not run for Senate or governor in 2010.

National and state Republicans are expected to quickly coalesce behind Kirk, a moderate who was first elected to the House in 2000. Democrats, meanwhile, appear headed toward a bloody primary fight, which could include state Treasurer Alexi Giannoulias, Chicago Urban League President Cheryle Jackson, businessman Chris Kennedy and appointed Sen. Roland Burris (D-Ill.), who still hasn’t indicated whether he will run for a full term.

Kirk is a prodigious fundraiser, and he has already amassed more than $1 million in his re-election account, which he can transfer for a Senate campaign. He recently announced that he had raised $580,000 in the second quarter of the year.

Kirk’s decision also ensures that the open-seat race to replace him in the 10th district will be among the most competitive of the cycle. The district favors Democrats, who will no doubt go all out to win the Chicago-based seat.