Sarah Palin resigns as Governor of Alaska.

Gov. Sarah Palin of Alaska abruptly announced on Friday that she was quitting at the end of the month, shocking Republicans across the country and leaving both parties uncertain about whether she was leaving national politics or laying the groundwork for a presidential run.

Ms. Palin, 45, the Republican vice-presidential nominee last year, was supposed to serve through the end of 2010; she said she would cede control of the state to Lt. Gov. Sean Parnell on July 26.

Speaking outside her home in Wasilla, Ms. Palin offered conflicting signals about her intentions and her motivation.

In her tone and some of her words in an often-rambling announcement, she sounded like someone who was making a permanent exit from politics after what her friends have called a rough and dispiriting year.

But her remarks, delivered in a voice that often seemed rushed and jittery, sounded at times like those of a candidate with continued national aspirations, as when she suggested she could “fight for all our children’s future from outside the governor’s office.”

Ms. Palin said that she had decided not to seek re-election when her term expires at the end of next year and that, given that, she did not think it was fair to her constituents to continue in office.

“As I thought about this announcement that I would not seek re-election,” she said, “I thought about how much fun other governors have as lame ducks. They maybe travel around their state, travel to other states, maybe take their overseas international trade missions.”

“I’m not going to put Alaskans through that,” she said. “I promised efficiencies and effectiveness. That’s not how I’m wired. I’m not wired to operate under the same old politics as usual.”

The news conference came at the end of a week in which a Vanity Fair article about Ms. Palin brought renewed focus on many of the criticisms of her as a candidate for vice president under Senator John McCain of Arizona and set off a new round of recriminations among Mr. McCain’s advisers about her competence.

But while Ms. Palin has been derided by much of the Republican elite, she remains extremely popular with many grass-roots members, especially social and religious conservatives.

Ms. Palin’s announcement was another unusual marker in what has been a tumultuous year for this first-term governor since Mr. McCain turned her into a national figure overnight by surprising his own party and naming her his running mate. It also underscored the instability in the Republican Party as it tries to find a strategy and voice in the wake of losses in 2008.

Ms. Palin made the announcement standing with her family. At one point she described how her children had voted in favor of her doing this " “Four yeses and one ‘Hell, yeah!’ ” she said " suggesting that the family had had enough of the scrutiny that had made them tabloid staples.

But at another point she invoked a military quotation, misattributing it to Gen. Douglas MacArthur, in what seemed to be an effort to wave aside any suggestion that she was abandoning the fight. “He said, ‘We’re not retreating; we are advancing in another direction,’ ” she said. (The remark was actually said by Maj. Gen. Oliver Prince Smith.)

Later in the afternoon, as questions reverberated in Republican circles about what exactly she intended to do, Ms. Palin posted a notice on her Twitter site, reading: “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.”

Ms. Palin is among a number of governors who are possible contenders for the Republican presidential nomination in 2012 and whose terms expire in 2010.

Many Republican strategists have argued that it would be difficult for someone to run for governor in 2010 and turn around immediately, while running a state, and run for president in 2012. Gov. Tim Pawlenty of Minnesota announced last month that he would not seek re-election when his term expired in 2010, as he considers a race for president.

Quitting midterm, however, is highly unusual. It set off speculation about what led her to leave so abruptly. One interpretation among Republicans was that she had simply underscored how erratic she is as a politician.

“Good point guards don’t quit and walk off the floor if the going gets tough,” said John Weaver, a former senior strategist for Mr. McCain. “Today’s move falls further into the weirdness category; people don’t like a quitter.”

But some of her supporters argued that this could actually provide Ms. Palin an opportunity to recover from what has been a damaging year for her, and prepare herself for the 2012 race. She has been enmeshed in continuing battles with members of both parties in her Legislature.