President Barack Obama has announced he'll run for office again in 2012, launching speculation about how much he'll spend. We've rounded up past incumbents who spared no expense in defending their political posts.

Barack Obama
The president has officially announced his 2012 re-election campaign, and some say it'll be the most expensive in history.
Michele Bachmann
Last November, U.S. Rep. Michele Bachmann, R-Minn., beat her challenger to retain this congressional district. The race earned a high-dollar distinction.
Barbara Boxer
In last year's U.S. Senate race, U.S. Sen. Barbara Boxer, D-Calif., beat her wealthy opponent. It's been reported that the senator spent nearly everything she'd raised
Michael Bloomberg
Bloomberg was named mayor of New York City the third time in a row (how did he do that?), having bested his 2009 challenger.
Harry Reid
Democrat Sen. Harry Reid went all out to secure his Nevada Senate seat last year. This meant aiming for a lofty goal. It was the second-most-expensive Senate race
George Pataki
Pataki was re-elected New York's governor in 2002, after spending somewhere in the ballpark of this large sum.
Hillary Rodham Clinton
Then-New York Sen. Hillary Rodham Clinton reportedly spent more than any other Senate candidate when she ran against her 2006 opponent.
John McCain
During the last midterm elections, Sen. John McCain, R-Ariz., is estimated to have spent more on a successful bid than any other Senate candidate.
Source:Specials