WASHINGTON - Sarah Palin, who surprised the U.S. political world last year by becoming Republican John McCain's vice presidential running mate, surprised again on Friday by announcing her resignation as Alaska's governor.

Palin, a former small-town mayor who became governor in December 2006, has been touted as a potential candidate for the Republican presidential nomination in 2012.

As McCain's Number Two, she ignited conservatives with her social views but turned off other voters who believed she was unprepared for the White House.

Having maintained her popularity at home in Alaska, why would she quit now? There are several possibilities:

* She frees herself from the constraints of daily governing so she can spend all her time pursuing national office. This would put her on a level playing field with potential Republican presidential primary opponents Mitt Romney, a former governor of Massachusetts, and Newt Gingrich, a former speaker of the U.S. House of Representatives. Another potential rival, Minnesota Governor Tim Pawlenty, also recently announced he would not seek re-election. Conversely, Palin's decision could diminish a selling point -- the fact that she has executive experience and knows how to run a state.

* She can make some money. Leaving her job would allow Palin to take a job in the private sector or take advantage of her popularity to bring in cash as a public speaker, author or talk-show host, possibly still making a run for national office.

* Palin could use the time to run for the U.S. Senate for Sen. Lisa Murkowski's seat in November 2010. Palin could use that job as a springboard for a presidential race.

* Palin can add to her credentials as a maverick, something both she and McCain emphasized during the 2008 campaign. Palin said she wanted to put Alaska first. She could use her decision to step down as evidence that she would not seek higher office while governor and so do harm to her home state.

* Palin may have grown tired of being a lightning rod for the American media and decided she wanted a break. The announcement was made late on a Friday before a U.S. holiday weekend -- an indication that she may have wanted to bury the news.

* She fears a looming political problem, perhaps even a scandal, and wanted out of the limelight, before the news broke. If there is any evidence that the decision was a result of political problems or looming scandals, she is done, said Julian Zelizer, a Princeton University history professor.

Sarah Palin