WASHINGTON (Reuters) - Newt Gingrich is quitting the U.S. Republican presidential contest after a tumultuous campaign marked by flashes of brilliance in debates, staff desertions and offbeat ideas.
Gingrich, a powerful figure in the 1990s as House of Representatives speaker, briefly reached the heights of Republican presidential front-runner this year, but stumbled badly under attack from rivals. The collapse of his campaign threatened to turn the proud intellectual into a punch line.
The final blow for his campaign came on Tuesday night when rival Mitt Romney easily won primary victories in five Northeastern states that crowned him as the presumptive Republican nominee.
Gingrich had campaigned heavily in Delaware as the conservative alternative to Romney, but lost by nearly 30 percentage points there.
Gingrich's idiosyncratic run turned ideas like establishing a U.S. moon colony and having schoolchildren work as janitors into front-page fodder. His campaign descended into near farce last week when he was bitten in the hand by a penguin during a visit to a zoo in St. Louis.
He will formally pull out of the race next week, a Gingrich campaign official said on Wednesday.
His withdrawal further clears the way for Romney, who has now claimed the unofficial mantle of the Republican nominee in November's election against Democratic President Barack Obama.
The only other Republican left in the race is U.S. Representative Ron Paul, a libertarian who is far behind Romney in polls and has not won a single nominating contest.
Clobbered by negative ads paid for by allies of Romney, Gingrich's campaign went downhill almost immediately after he won the South Carolina primary on January 21. He flopped in the following primary in Florida and never recovered.
A bombastic campaigner who shed tears talking about his mother in Iowa, Gingrich enjoyed playing against type on the campaign.
He was strongest in televised presidential debates where he unleashed slashing attacks on Republican rivals, Obama, and even debate moderators whom he suspected of liberal media bias.
But he won only two states in the primary season and made the decision to quit after receiving an early morning phone call on Wednesday from Romney, said Gingrich spokesman R.C. Hammond.
Gingrich agreed to endorse the former Massachusetts governor next week, although he has no aspirations to serve in a future Romney administration, Hammond said.
"Newt's next role in life is as a citizen," Hammond said.
Gingrich plans to assist efforts to maintain Republican control of the House and help Republicans seize a majority in the Senate, Hammond said.
Before his presidential run, Gingrich remained in the public eye through television appearances, constant book publication, and his personal empire of research and advocacy organizations.
Since Gingrich began his campaign, some of those organizations faltered. A healthcare think tank Gingrich started declared bankruptcy this month. Another outlet once available to Gingrich may now be closed: Gingrich has grown critical of Fox News, his former employer.
Supporters say Gingrich's many policy ideas will keep him relevant.
"He's always going to be an intellectual leader," said South Carolina Republican operative Katon Dawson, who advised the early stages of Gingrich's campaign.
The 68-year-old seemed to enjoy himself to the end, when he scaled back on campaign rallies in recent weeks and visited zoos and museums while on the road.
"I never got the sense that he was quote-unquote down," said adviser Charlie Gerow. "I got the sense on a couple of occasions that he was tired. Really tired."
Gingrich provided a lesson in the power of the biggest force in this year's contest: the newly arrived independent "Super PACs," political action committees that have no limits on how much money they can raise or spend in support of candidates.
Throughout the primary season, Gingrich depended on the largesse of a Super PAC called Winning Our Future, which received at least $21.5 million in donations from billionaire casino mogul Sheldon Adelson and his family.
At the same time, a pro-Romney PAC, Restore Our Future, spent $19 million on ads attacking Gingrich.
The Romney camp criticized him as a Washington insider, accusing Gingrich, who rose to be the most powerful U.S. Republican during his leadership of the House in the 1990s, as tainted by the ways of the nation's capital.
Gingrich's lucrative contract with mortgage giant Freddie Mac helped the Romney campaign makes its case. Ironically, Gingrich, criticized as the consummate Washington creature, struggled to secure the support of many of the lawmakers he once led.
His White House bid got off to a bad start last June when almost 20 staff members resigned in anger at the campaign's lack of organization and the fact that the candidate went on a Greek cruise with his wife instead of campaigning or fundraising.
In its last days, Gingrich hardly had a campaign at all.
A past state director for Gingrich said Monday the pace at Gingrich's Virginia headquarters had slowed. Where 50 staffers once worked, a dozen appeared, many leaving their desks at 5 p.m. each day - a rarity in a profession known for its long work days.
Debts piled up. Last week, the campaign declared it had spent $4.3 million more than it had raised.
In part, Gingrich believed his White House bid faltered because the country was not prepared for his big ideas.
"I haven't done a very good job as a candidate because it's so difficult to communicate big solutions in this country when the entire structure of the system is so hostile to it," he told students at Georgetown University last month.
His candidacy never lacked drama. Days before Gingrich's victory in South Carolina, his second wife appeared in a television interview to accuse Gingrich of asking her for an open marriage while he was having an affair with his current and third wife, Callista.
Gingrich vociferously denied the charge at a debate, the forum that helped lift his campaign.
Despite Gingrich's spirited persona, underscored by his feisty debate performances, he had a warmer side. He made sure to remember birthdays for members of his traveling press corps and liked to joke that his grandchildren, Robert and Maggie, were his "senior debate coaches."
Family also caused Gingrich trouble: Callista's smiling, often silent presence at campaign events offered a constant reminder of Gingrich's complicated private life, which turned some evangelical voters away from him.
Additional reporting by Steve Holland; Editing by Alistair Bell and Peter Cooney