An aggressive evening debate in the critical state of South Carolina, site of Saturday's primary vote, capped the bewildering day, which also saw Romney's narrow win in the lead-off Iowa caucuses turn into a narrow defeat.
Recent polls, coupled with Perry's endorsement, suggested Gingrich was the candidate with the momentum and Romney the one struggling to validate his standing as front-runner.
Whatever else the impact, the day's events reduced the number of contenders vying to emerge as Romney's principal conservative alternative. Conservatives have not coalesced behind Romney, who has shifted positions on several key social issues since he was governor of the Democratic state of Massachusetts.
The debate began a few hours after the first word that Romney's narrow Iowa caucus victory was actually a narrow loss, then had been stung by Perry's endorsement of Gingrich.
Gingrich, in turn, was accused by an ex-wife of seeking an open marriage so he could keep his mistress.
"Newt's not perfect, but who among us is," said Perry, abruptly quitting the race just before the first-in-the-South primary.
His decision to end a once-promising candidacy left Romney, Gingrich, former Sen. Rick Santorum and Texas Rep. Ron Paul the remaining contenders in the race to pick a Republican to challenge Democratic President Barack Obama this November.
Hours after Perry exited one stage, the four remaining contenders walked onto another for a final pre-primary debate.
Gingrich angrily denounced the news media for putting his ex-wife front and centre just before South Carolina votes, but Santorum, Romney and Paul steered well clear of the controversy.
The audience gave Gingrich a standing ovation when he assailed the media, a reaction he can only hope is reflected in voter sentiment on Saturday.
Santorum mentioned his change of fortunes in Iowa, where an evident eight-vote defeat in caucuses on Jan. 3 was belatedly transformed into a 34-vote advantage on Thursday, though the Iowa Republican Party has yet to officially declare a winner. Romney went on to easily win the New Hampshire primary.
Santorum played aggressor for much of the night, struggling to inject himself into what seemed increasingly like a two-way race. He accused the surging Gingrich and front-runner Romney of agreeing with the left when it came to health care. Both men rejected the allegations.
All four remaining Republican candidates lustily attacked Obama, but first Gingrich and then Santorum challenged Romney over his well-documented switch of position on abortion. Romney now says he opposes abortion.
Romney refused to release his income tax returns before the weekend's vote and adamantly refused to explain why some of his millions were invested in the Cayman Islands, how much was there or whether any other funds were held offshore.
Gingrich grappled with problems of a different, possibly even more crippling sort in a state where more than half the Republican electorate is evangelical Christian.
In an interview scheduled to air on ABC News, Marianne Gingrich said her ex-husband had wanted an "open marriage" so he could have both a wife and a mistress. She said Gingrich conducted an affair with Callista Bistek — his current wife — "in my bedroom in our apartment in Washington" while she was elsewhere.
"He was asking to have an open marriage and I refused. That is not a marriage," she said in excerpts released by the network in advance of the program.
Asked about it during the debate, Gingrich repeatedly criticized the media, then briefly addressed the accusation. "The story is false," he said, with no elaboration.
Santorum, whose fortunes have ebbed since what appeared to be a narrow loss in Iowa, touted his newly declared victory there.
"There have been two contests. We won one," he said, and he proceeded to ridicule Romney and Gingrich as weak challengers to Obama. "How can you differentiate ourselves on the major issues of the day if we nominate tweedledum and tweedledee instead of someone who stood up and said, 'No'?" he said to one audience, referring to his opposition to a requirement to purchase health care coverage.
Perry's exit marked the end of a campaign that began with soaring expectations but quickly faded. He shot to the head of the public opinion polls when he announced his candidacy last summer, but a string of poor debate performances soon led to a decline in support.
Associated Press writers David Espo, Thomas Beaumont, Beth Fouhy, Philip Elliott, Kasie Hunt and Shannon McCaffrey in South Carolina contributed to this story.
Good grief what does it take to demonstrate just how incompetent these
people really are?