Rep. Mark Kirk (R) began telling supporters on Wednesday that he will run for Senate in 2010, setting up what is expected to be a blockbuster contest and also touching off a highly competitive open-seat race to succeed him.

Kirk’s move followed news that Illinois Attorney General Lisa Madigan (D) had decided to seek re-election rather than running 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. Kirk is a prodigious fundraiser, and he has already amassed more than $1 million in his re-election account, which he can transfer to a Senate campaign. He recently announced he had raised $580,000 in the second quarter of the year.

Democrats, meanwhile, appear headed toward a bloody primary fight, which already includes state Treasurer Alexi Giannoulias and Chicago Urban League President Cheryle Jackson. Plus, businessman Chris Kennedy is expected to enter the race in the coming weeks. Sen. Roland Burris, who was appointed to fill President Barack Obama’s unexpired term, hasn’t indicated his plans but is unlikely to run again for a full term in 2010.

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 North Shore Chicago seat.

Marketing consultant Dan Seals (D), who lost to Kirk in 2006 and 2008, is all but certain to enter the race now. State Sen. Michael Bond (D) has already announced that he is running. State Sen. Susan Garrett (D) has also been mentioned as a possible candidate.

Among Republicans, state Rep. Beth Coulson, a 13-term state legislator, is likely to consider the race. However, Coulson is considered to be even more politically moderate than Kirk on some issues — which would be a problem in a contested GOP primary.

Businessman Dick Green is also interested in running, according to local Republican sources.