WASILLA, Alaska — Alaska Gov. Sarah Palin surprised supporters Friday and announced she is resigning from office at the end of the month, leaving open the possibility she would seek a run for the White House in 2012.

The news rattles a Republican Party plagued with setbacks in recent weeks, including extramarital affairs disclosed by two other 2012 presidential prospects, Nevada Sen. John Ensign and South Carolina Gov. Mark Sanford. Palin and her staff kept her future plans shrouded in mystery, and it was unclear if the controversial hockey mom would quietly return to private life or begin laying the foundation for a presidential bid.

Palin hinted she had a bigger role in mind, saying she wanted to make a “positive change outside government.” But she kept supporters in suspense, promising on Twitter: “We’ll soon attach info on decision to not seek re-election ... this is in Alaska’s best interest, my family’s happy ... it is good. Stay tuned.”

In a hastily arranged news conference at her home in suburban Wasilla, Palin said she will formally step down July 26, and Lt. Gov. Sean Parnell will be inaugurated at the governor’s picnic in Fairbanks. She said she had decided against running for re-election as Alaska’s governor, and believed it was best to leave office even though she had two years left to her term.

“Many just accept that lame duck status, and they hit that road. They draw a paycheck. They kind of milk it. And I’m not going to put Alaskans through that,” she said.

The 2008 vice presidential nominee was seen as a likely presidential contender in 2012 and had proved formidable among the party’s base. But the last week brought a highly critical piece in Vanity Fair magazine, with unnamed campaign aides questioning if Palin was ever really prepared for the presidency.

The backbiting continued through the week, with follow-up articles recounting the nasty infighting that plagued her failed bid. Her advisers sniped with other Republicans, underscoring the deeply divided GOP looking for its next standard bearer.

Palin’s resignation, timed on the eve of the July 4 holiday when many Americans had already begun a three-day weekend, seemed designed to avoid publicity while openly leaving office. She alluded to how she could help change the country and help military members — code that she didn’t think her time on the national stage was over.

Palin’s spokesman, David Murrow, said the governor didn’t say anything to him about this being her “political finale.” He said he interpreted Palin’s comment about working outside government as reflecting her current job only.

“She’s looking forward to serving the public outside the governor’s chair,” he said.

And Pam Pryor, a spokeswoman for Palin’s political action committee SarahPAC, said the group continues to accept donations on its Web site, with an uptick in funds after Palin’s announcement.

Jerry McBeath, a veteran political science professor at the University of Alaska Fairbanks, called the pending resignation a “smart move,” both for Palin and the state.

“Alaska is an isolated stage from which to operate if you want to figure in American national politics. I don’t know what she has in mind. Some TV show or some national radio show. There are opportunities for her, I’m sure.”

But political analyst Larry Sabato, in Charlottesville, Va., said Palin’s announcement left many confused.

“It’s absolutely bizarre, and I think it eliminates her from serious consideration for the presidency in 2012,” he said.

Palin said her family weighed heavily in her decision.

“I polled the most important people in my life, my kids, where the count was unanimous,” she said. “Well, in response to asking, ‘Hey, you want me to make a positive difference and fight for all our children’s future from outside the governor’s office?’ It was four yeses and one ‘Hell, yeah!’ And the ‘Hell, yeah’ sealed it.”

Palin’s decision not to seek re-election was a familiar one for a potential presidential candidate. Former Massachusetts Gov. Mitt Romney chose not to seek another term as he geared up for an unsuccessful 2008 presidential bid. Minnesota Gov. Tim Pawlenty has announced he won’t seek another term, giving him plenty of free time ahead of a potential 2012 bid.

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 updo and folksy “You betcha!” on “Saturday Night Live.”

In the presidential race, Palin became the butt of talk-show jokes and Democratic criticism after news broke that the Republican Party had spent $150,000 or more on a designer wardrobe, accessories and hair and makeup services for her. The high-end spending spree contrasted with the down-to-earth image she sought to craft for herself and became an unwelcome issue for the McCain campaign.

She didn’t leave the limelight once McCain lost the presidency. She recently 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 also 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.

Fred Malek, a Republican strategist who has advised Palin over the past year, said Palin was “really unhappy with the way her life was going.”

“She felt that the pressures of the job combined with her family obligations and the demands and desires to help other Republican candidates led her to decide not to run again. Once that decision was made, she realized, why not do it now and let the lieutenant governor take over and get a head start on his election,” Malek said.

Palin was first elected in 2006 on a populist platform. But her popularity has waned as she waged in partisan politics following her return from the presidential campaign. Her term would have ended in 2010.

Palin expressed frustration with her current role as governor.

“I cannot stand here as your governor and allow the millions of dollars and all that time go to waste just so I can hold the title of governor,” Palin said.

Her 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’s announcement comes after several recent blows to the Republican party. Ensign, a member of the Christian ministry Promise Keepers, stepped down from the Senate Republican leadership last month after admitting he had an affair for much of last year with a woman on his campaign staff who was married to one of his Senate aides. Ensign later disclosed he had helped the woman’s husband get two jobs during the affair.

A government watchdog group, Citizens for Responsibility and Ethics in Washington, wants the Senate ethics committee and the Federal Election Commission to investigate.

Just days after news of Ensign’s affair broke, Sanford admitted an affair with a woman in Argentina. Some lawmakers are now calling for his resignation. Before the admission, Sanford had been missing from the state for five days visiting his lover. He had slipped his security detail, lied to his staff about where he was and failed to transfer power to the lieutenant governor in case of a state emergency.

The party’s troubles seem to have left two prominent 2012 prospects, former House Speaker Newt Gingrich and 2008 presidential hopeful Mitt Romney, unscathed, however.