Barack Obama on Jan. 21, 2009. Credit: Pete Souza

"Today, we are filing papers to launch our 2012 campaign,“ Obama noted in a message to supporters. “We're doing this now because the politics we believe in does not start with expensive TV ads or extravaganzas, but with you -- with people organizing block-by-block, talking to neighbors, co-workers and friends. And that kind of campaign takes time to build. So, even though I'm focused on the job you elected me to do, and the race may not reach full speed for a year or more, the work of laying the foundation for our campaign must start today.”

The Obama team launched a video and  a redone campaign website Monday.

While the video focuses on Obama backers and why they support the president’s bid for a second term, the website does have one telling feature: It includes a reference to “Obama-Biden.” While there has been speculation that Vice President Joe Biden would be replaced on the Democratic ticket,  Secretary of State Hillary Clinton, the most likely option to take his place, announced last month that she has no interest in the position.

Obama’s announcement came as no surprise. With a handful of exceptions -- James K. Polk, James Buchanan, Rutherford B. Hayes, Calvin Coolidge -- most presidents who are eligible have sought additional time in office. Unlike Gerald Ford, Jimmy Carter and George H. W. Bush, the last three presidents who were denied re-election, Obama has not drawn a serious challenger for the Democratic nomination, though pro-life activist Randall Terry -- founder of Operation Rescue -- has thrown his name into the mix, and former U.S. Sen. Mike Gravel of Alaska, who ran for the Democratic nomination against Obama in 2008, has left the door open for another bid.

Republicans quickly responded to the announcement. Former Gov. Tim Pawlenty of Minnesota, who has formed an exploratory committee to seek the Republican presidential nomination, released a video of his own responding to the news.