U.S. Sen. Herb Kohl announced today that he will step down when his term expires in 2012, ending a Senate career of more than 20 years.

Kohl was first elected in 1988 and easily won re-election three times, thanks mostly to his tremendous financial advantage. In his last election in 2006, Republican challenger Robert Lorge got only 29 percent of the vote.

But the open seat could make next year's Senate election one of the most hotly-contested in the nation.