Mr. Gingrich joins the race amid a shifting political landscape, altered 10 days ago when President Barack Obama announced that Osama bin Laden had been killed in Pakistan. Mr. Obama’s approval rating hit its highest point in two years, 60 percent, and more than half of Americans now say he deserves to be re-elected.

Republican Newt Gingrich, the former leader of the House of Representatives known for his combative speech and turbulent tenure in Congress, formally announced Wednesday that he will attempt a political comeback by running for president.

In a post on Twitter Wednesday, made official what has been an open secret for months- “Today I am announcing my candidacy for president of the United States.”

Mr. Gingrich joins the race amid a shifting political landscape, altered 10 days ago when President Barack Obama announced that Osama bin Laden had been killed in Pakistan. Mr. Obama’s approval rating hit its highest point in two years, 60 percent, and more than half of Americans now say he deserves to be re-elected, according to an Associated Press-GfK poll released Wednesday.

In 1994 Mr. Gingrich led Republicans to their first majority in the House in four decades and then served four tumultuous years as speaker, as the House leader is known.

Mr. Gingrich later acknowledged having carried on an affair with a congressional aide, now his third wife, at the same time he was criticizing President Bill Clinton for his relationship with White House intern Monica Lewinsky. Mr. Gingrich’s marital history could mean trouble with conservative Christians who turn out in big numbers for the party’s nominating primaries.

As speaker, Mr. Gingrich made the sort of deals that current conservative activists say they disdain- creation of a new health care benefit program as part of a balanced budget agreement with Clinton, for example.

The 67-year-old former Georgia congressman will be among the oldest candidates - if not the oldest - in a still-forming Republican field of politicians with far more recent political experience. Among those considering bids are Rep. Michele Bachmann and former Alaska Gov. Sarah Palin, two darlings of the ultraconservative tea party movement.

In a YouTube video, Gingrich assures viewers, “There’s a much better American future ahead with more jobs more prosperity, a better health system, longer lives, greater independent living and a country that is decentralized under the 10th Amendment with power once again back with the American people and away from the Washington bureaucracy.”

Mr. Gingrich struck a combative tone, too, saying at one point- “There are some people who don’t mind if America becomes a wreck as long as they dominate the wreckage.” He also said, “We Americans are going to have to talk together, work together, find solutions together and insist on imposing those solutions on the forces that don’t want to change.”

In an interview on Fox News, he came out swinging against Mr. Obama, accusing the Democrat of using “scare tactics” to attack a Republican budget proposal and the party’s views on immigration reform.

“President Obama should be ashamed of himself,” Mr. Gingrich said. He assailed Mr. Obama’s foreign policy as “a very, very discouraging mess” and labelled his energy policy “anti-American”.

After a party setback in the November 1998 elections, Mr. Gingrich announced abruptly he would be stepping down as speaker and, though re-elected by his district in Georgia, would retire from the House. He left in the face of a leadership challenge, with some Republicans disillusioned with his temperament.

During his speakership Gingrich was reprimanded by the House in 1997 and agreed to pay a $300,000 assessment for the costs of an investigation into questions about the political use of a tax-exempt college course he offered.

Mr. Gingrich had been publicly flirting with the race for months and sparked stories about his candidacy days before the official announcement. He said Monday he would seek the Republican nomination, telling fans to tune into Fox News, where he had been a paid commentator for years, to hear a talk about his presidential run.

One of the best-known images of Gingrich from his days as speaker was the New York Daily News cover depicting him in a diaper pitching a tantrum after being barred from sitting up front with President Clinton on Air Force One while returning from Israel. And his latest outbursts have raised eyebrows, including when he compared a mosque that was to open near the site of the destroyed World Trade Center towers in New York to Nazis putting a sign next to the Holocaust Museum.

As he geared up for a campaign, Mr. Gingrich allies have privately urged him to tone down the bomb-throwing rhetoric, arguing he should strive to be seen as the adult in the room presenting the battle-tested big ideas rather than a conservative firebrand.

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