Palin's decision even took Parnell by surprise. He said he was told on Wednesday evening, and was not aware that any presidential ambitions were behind the move.

Palin emerged from relative obscurity nearly a year ago when she was tapped as then Republican presidential candidate John McCain's running mate.

She was a controversial figure from the start, with comedian Tina Fey famously imitating her elaborate hairstyle and folksy "You betcha!" on "Saturday Night Live."

Most recently, she led a public spat with "Late Show" host David Letterman over a joke he made about one of her daughters being "knocked up" by New York Yankees baseball player Alex Rodriguez during the governor's recent visit to New York. Palin's 18-year-old daughter, Bristol, is an unwed, teenage mother.

Letterman later apologized for the joke.

Palin's family and the ridicule they endure being in the public eye was part of her decision. She complained that her 14-month-old son, Trig, who was diagnosed with Down's syndrome, had been "mocked and ridiculed by some mean-spirited adults recently." She didn't elaborate.

Palin campaigned on ethics reform in the 2006 election, defeating incumbent Gov. Frank Murkowski in the Republican primary and a former two-term Democratic governor, Tony Knowles, in the general election.