|Gingrich has lambasted Obama's federal health care reform law and criticised his foreign policy as "clueless" [Getty]
Newt Gingrich, former Speaker of the US House of Representatives, appears set to join the race for the Republican presidential nomination to challenge Barack Obama in 2012.
The deeply conservative Gingrich ranks among the best-known Republicans in the still-forming group of candidates for next year's contest for the White House.
"I have been humbled by all the encouragement you have given me to run," Gingrich wrote on Monday in his official announcement on Facebook and Twitter. He said he would talk more about his bid on Wednesday on Fox News.
Gingrich also brings considerable negative political baggage: three marriages, a resignation under an ethics cloud while leader of the House, and a tendency to shoot from the hip when speaking.
In 1994, Gingrich led the Republican Party to its first House majority in 40 years. But any Republican candidate could face insurmountable difficulties in defeating the incumbent Obama, who remains personally popular with Americans and has seen his job approval rating rise notably since last week's killing of al-Qaeda chief Osama bin Laden.
Obama is expected to raise as much as $1bn to finance his bid for re-election, and has proven himself a highly effective campaigner.
History would suggest a difficult path for Gingrich. While many former presidents and candidates for that office have served in the House, the last president to have served as House Speaker was James K Polk, a Tennessee Democrat, in 1844.
Successful presidential candidates more often move into the job from the vice presidency, the Senate or the governorship of one of the 50 states.
Gingrich has made no secret of his White House ambitions. He has been raising money and assembling a campaign team for months and has travelled frequently to states that hold early presidential primary elections or caucuses.
Doing well in early primaries is considered necessary for a candidacy to prosper.
Getting into the race marks a comeback attempt by the former congressman from Georgia who stepped down from the House after four tumultuous years in the top position as speaker.
A spending fight between Gingrich and president Bill Clinton led to a shutdown of part of the federal government in 1995 and 1996. He left Congress in 1999.
Since then, he has established a network of nonprofit and lucrative business ventures. He also has churned out a steady stream of books and made frequent speaking engagements.
In recent months, Gingrich has lambasted Obama's federal health care reform law, and has criticised the Democrat's foreign policy as "clueless".
Gingrich is dogged by extramarital affairs and having been married three times. He has been working to make inroads with social conservatives critical to the Republican primary base, highlighting his conversion to Roman Catholicism after marrying his third wife, Callista.
He has also been criticised as a glib political figure who is not long on consistency in public statements.
He calls for a muscular approach to combating terrorism. But he was widely mocked recently for an about-face on Libya policy.
First he said he would "exercise a no-fly zone" and get rid of Muammar Gaddafi. Two weeks later, he said: "I would not have intervened. ... I would not have used American and European forces."
The early running for next year's elections has been highly unusual in that possible Republican candidates have been very late in announcing their intentions.
While the initial field is expected to be crowded, it is more divided than usual, given that the Republican mainstream faces a splintering off of ultraconservative Tea Party factions and others with populist or libertarian messages.
According to Gallup polling records dating back to 1952, when General Dwight D Eisenhower was the party standard-bearer, the Republicans have never been without a leading candidate at this stage in the campaign cycle.
So far and in addition to Gingrich, only former Massachusetts governor Mitt Romney, former Minnesota governor Tim Pawlenty and former Pennsylvania senator Rick Santorum have taken the first official step by forming campaign exploratory committees.
About a dozen Republicans are believed to be contemplating a candidacy.