Former Republican vice-presidential candidate Sarah Palin has announced she will resign as governor of Alaska on 26 July and not run for re-election.
Mrs Palin's term of office was due to end in 2010.
Some have speculated that Mrs Palin, who is popular with the Republican Party base, might be preparing to make a bid for the White House in 2012.
But a report on NBC news suggested that Mrs Palin intends to get "out of politics for good".
Her resignation means Alaska's Lieutenant Governor Sean Parnell will take over as state governor.
Polls indicated Mrs Palin was very popular in Alaska during the first few years of her governorship, and although her approval ratings have dipped somewhat since her vice-presidential run, she still enjoys widespread popularity in her home state.
Mrs Palin announced her decision in a statement from her home town of Wasilla, Alaska.
"I'm taking my fight for what's right in a new direction," she said, as her family looked on.
Mrs Palin did not reveal what she intended to do after leaving office, and did not give an explicit reason for her decision not to run for re-election.
But in a written statement, she made it clear that once she had decided not to run again, she did not want to hang on in office until her term expired.
"Once I decided not to run for re-election, I also felt that to embrace the conventional Lame Duck status in this particular climate would just be another dose of politics as usual, something I campaigned against and will always oppose," she said.
The BBC's Kevin Connolly in Washington says Mrs Palin's revelation came out of the blue, as most Americans were turning to the celebration of Independence Day on 4 July.
She offered no single clear reason for stepping down, our correspondent adds, but the strongest clue was her depiction of what it had been like to be the subject of sustained attack by liberals since she appeared on the national stage.
US MEDIA REACTIONS TO PALIN'S RESIGNATION
If she does decide to run for future office, Palin will now face the challenge of explaining to voters why she should be president of the United States despite serving less than three of the four years of her elected term as Alaska's governor, and spending months of her second year as governor campaigning for the vice presidency.
Listening to her, it seems like this is a combination of stepping back and moving forward. Stepping back, because it's way too overwhelming to be Sarah Palin, political phenom, Sarah Palin, governor of Alaska, and Sarah Palin, wife and mother. I don't know that anyone can fulfil all those roles well, simultaneously. And we're unrealistic, I think, when we assume people can or should.
I think the simple truth is that, as even Alaskan Republicans told us last September, she was far from able to be governor of Alaska, let alone vice-president of the United States. Once the klieglights hit, it was only a matter of time before she imploded or exploded or some gruesome combination of the two.
If Palin wants to run in 2012, why not do exactly what she announced today? It's an enormous gamble - but it could be a shrewd one. After all, she's freeing herself from the duties of the governorship. Now she can do her book, give speeches, travel the country and the world, campaign for others, meet people, get more educated on the issues - and without being criticized for neglecting her duties in Alaska.
Not finishing her first term will provide a major, major, major obstacle to any presidential bid. I thought a 2012 campaign would be a mistake; from today's comments, it's not clear whether Palin is still interested in that option.
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