After Sen. Herb Kohl announced his retirement, President Obama called the Wisconsin Democrat to thank him for his many years of service.

"During his 23 years in the United States Senate, Herb’s invaluable perspective as the long-time head of a family-owned business made him an unwavering voice for working families, small business owners, and seniors," the president said in a statement released by the White House.

"America’s children will grow up in a better place thanks to his advocacy on behalf of childhood nutrition programs, a strengthened food safety system,  access to affordable health care and childcare, and juvenile crime prevention."

Kohl, the quiet Wisconsin senator and owner of the NBA's Milwaukee Bucks, will depart from the Senate after four terms, setting up what could be a tough battle for his seat.

The two candidates widely considered to be the top prospects to run for Wisconsin's newly open Senate seat — GOP Rep. Paul Ryan and former Democratic Sen. Russ Feingold — both left open the possibility of a 2012 bid on Friday.

Kohl becomes the sixth senator in the Democrats' corner to decline reelection next year; Daniel Akaka (Hawaii), Jeff Bingaman (N.M.), Kent Conrad (N.D.) and Jim Webb (Va.) have said they'll retire at the end of this term, as will Sen. Joe Lieberman (Conn.), an Independent who caucuses with Democrats.