UPDATE, 6:35 p.m.: Sarah Palin is not running for president.

Ms. Palin, the former governor of Alaska, ended her inscrutable cat-and-mouse game with the political establishment on Wednesday afternoon by saying that she would not join the field of Republican candidates seeking her party’s nomination, but would still work to oust President Obama.

“Not being a candidate, you are unshackled and able to be even more active,” she said during a “Mark Levin Show” appearance. “I look forward to using all the tools at my disposal to get the right people in there who have a servant’s heart.”

The decision from Ms. Palin on Wednesday placed a punctuation mark on the Republican presidential field. Her intentions were the biggest remaining question in the race. While there was not a groundswell of support among Republican leaders for a Palin candidacy, she has a deeply loyal following.

Her announcement, which took the Republican world by surprise, was vintage Palin. It also raised the question of how intensely she plans to be involved in the race and whether she intends to endorse a candidate or simply be a commentator, cheerleader or critic.

For a year, Ms. Palin had given many signals that she would not run. She has repeatedly suggested that she could have a bigger influence if she was not a candidate. But her supporters were convinced that she was running. A Labor Day weekend appearance in Iowa drew boosters from across the country, most of whom said they were certain she would run.

The immediate effect of her decision to stay on the sidelines was minimal. Yet the long-term impact is less certain. As she demonstrated in the successful Republican midterm elections last year, her endorsement was a hot ticket.

Her late decision comes a day after Gov. Chris Christie of New Jersey announced that he, too, would take a pass on a presidential bid.

Ms. Palin, who rocketed to fame as the Republican vice-presidential nominee in 2008, said she still intended to use her celebrity status to wield political influence without the support of a Republican infrastructure that is wary of her powerful but unorthodox appeal.

Speaking on the radio show, Ms. Palin said that she intended to continue waging a “mission to help wake up America to what’s going on in this country.”

“I can be more effective and more aggressive in this mission” without being a candidate, she said.

“I need to be able to say what I want to say,” she added, and to “call out the wrongheadedness of our leaders who are leading us down this path. Let us unite to restore this country.”

In an e-mail sent to her supporters, Ms. Palin said she had decided not to run out of respect for her family and the impact that a campaign would have on them.

“As always, my family comes first and obviously Todd and I put great consideration into family life before making this decision,” she wrote, referring to her husband. “When we serve, we devote ourselves to God, family and country. My decision maintains this order.”

She said in the e-mail that she would remain politically active.

“I will continue driving the discussion for freedom and free markets, including in the race for president where our candidates must embrace immediate action toward energy independence through domestic resource developments of conventional energy sources, along with renewables,” she wrote. “We must reduce tax burdens and onerous regulations that kill American industry, and our candidates must always push to minimize government to strengthen the economy and allow the private sector to create jobs.”

Gov. Rick Perry of Texas, whose candidacy could benefit the most from her decision, called Ms. Palin “a good friend, a great American and a true patriot.”

“I respect her decision and know she will continue to be a strong voice for conservative values and needed change in Washington,” he said in a statement.

Aides to Ms. Palin have said for months that a Palin campaign would have largely ignored the usual political traditions in favor of an effort designed to communicate directly with the public through Ms. Palin’s Facebook page and Twitter feed, which combine to reach millions of followers.

Ms. Palin has used those forums, as well as her contract as a prominent Fox News analyst, to maintain her position as one of the leading conservative voices in the Republican Party during the last several years. In the 2010 midterm elections, her endorsement served as a powerful indication of support from the Tea Party movement, which Ms. Palin has adopted has her own.

She has also become one of the fiercest critics of Mr. Obama’s policies at a time that Republican voters are longing for someone who can reclaim the White House from the Democratic Party.

“Barack Obama has shown us cronyism on steroids,” she said in and Iowa speech on Labor Day. “It will lead to our downfall if we don’t stop it now. The challenge is not simply to replace Obama in 2012. The real challenge is who and what we will replace him with. It’s not enough to just change up the uniform. If we don’t change the team and the game plan, we won’t save our country.”

The question of whether Ms. Palin would join the 2012 presidential race had been one of the most intriguing — and hotly debated — among veteran political observers in Washington. Many assumed that she would decide against a bid because of her financial success as a best-selling author, political commentator and reality TV star.

There was also the experience of the 2008 campaign, which served as a brutal entry onto the national political stage for Ms. Palin and her family. In three intense months, her intelligence was mocked, her family was scrutinized and she developed a searing distrust of political consultants and the national press corps.

Senator John McCain, Republican of Arizona, who picked Ms. Palin to be his vice-presidential nominee in 2008, went to Twitter Wednesday evening to comment: “I am confident she’ll continue to play an important role in our Party and for our nation.”

Until her announcement, there have been plenty of clues that Ms. Palin was intent on keeping alive the possibility of a presidential campaign.

Her two best-selling books served as responses to the mockery she received as a candidate for vice president and as the beginnings of a platform from which she could build a political identity separate from Senator McCain.

She continued both efforts as a highly-paid public speaker — reportedly earning as much as $100,000 a speech — and as a highly paid analyst for Fox News. She also built an online presence that rivals almost every other national politician. Her commentary on Twitter and Facebook regularly get more attention than most officeholders.

And earlier this year, Ms. Palin began a thinly veiled test of what it might be like to campaign for president by taking her family on visits to historic American landmarks in her “One Nation” bus tour.

Ms. Palin insisted that the bus tour was more family vacation than political exercise. But her small circle of aides made little effort to disabuse journalists of the idea that the bus tour was designed to lay the groundwork for a possible campaign.

The tour also proved Ms. Palin’s devilish side. She took the bus into New Hampshire just as Mitt Romney formally announced his campaign in the state. And she suddenly arrived at the Iowa State Fair on the day before the Republican straw poll in Ames.

A campaign would have tested her political appeal as she began campaigning alongside her rivals for the nomination, Michele Bachmann, Mr. Romney and Mr. Perry. There are barely three months until the voting begins in Iowa, New Hampshire and South Carolina early next year, and Ms. Palin would have started with almost no formal campaign infrastructure in any of those states.

A campaign would have tested Ms. Palin’s ability to quickly raise millions of dollars necessary to finance a modern presidential campaign. Those closest to her have said they believed donations from her many admirers around the country would have rapidly poured in over the Internet.