WASHINGTON // Newt Gingrich's victory in South Carolina on Saturday has blown open the Republican race to challenge Barack Obama for the US presidency in November.
Mr Gingrich's decisive win prompted his primary rival, Mitt Romney, to announce yesterday that he will release his 2010 tax returns and 2011 estimates tomorrow, acknowledging it was a mistake for his campaign not to have done so earlier.
The former Massachusetts governor and venture capitalist said it was "not a good week for me" and he cited the tax-return issue.
After months of resistance, Mr Romney had said last week that he would release tax information for 2011, but not until April.
That was seen as a time, before the South Carolina race rattled his front-runner status, when the GOP nomination might have been decided.
Mr Gingrich has changed that perception, partly with his effective performances in the campaign debates. His aggressive style turned the race around in South Carolina, a deeply conservative state, and he won by a 41 to 27 per cent margin over Mr Romney.
Three states have now cast their votes; three of the four remaining candidates have won one state apiece. But Mr Gingrich's victory Saturday is the most significant.
Rick Santorum, the former Pennsylvania senator and Mr Gingrich's rival for the Republican Party's most conservative members, has vowed to fight on, but his challenge appears to be fading. On Thursday he was declared the winner of the Iowa race by a tiny margin over Mr Romney but it has not been enough for him to build much momentum.
Ron Paul, the Texan Representative, is likely to carry on until the end in an effort to garner enough votes to influence the party platform that is adopted at the Republican National Convention in August. But Mr Paul's is almost a parallel race. He continues to garner a consistent percentage of the votes but not nearly enough to win the race.
Mr Gingrich, former leader of the House of Representatives, has been helped in no small part by a barrage of negative advertising targeting Mr Romney.
The central theme of that advertising questioned how Mr Romney made his money with Bain Capital, a venture capital firm. Part of that attack focused on Mr Romney's refusal to release his tax returns.
From the beginning in Iowa, it was clear that Republicans were looking for an anti-Romney. A Mormon, Mr Romney has had difficulty collecting the Evangelical Christian vote, a significant bloc in the Republican Party.
He also has suffered from what some Republicans consider a liberal history in office in Massachusetts. As governor there, Mr Romney passed healthcare reform that was partly a model for the controversial reforms Mr Obama promoted.
Moreover, he is accused of flip-flopping on issues close to Republican hearts, from abortion to gun control.
The only question was who could successfully harness those doubts about Mr Romney, a question Mr Gingrich's victory seems to have answered.
Mr Gingrich ran an astute campaign appealing to national security-orientated conservatives by touting small government at home and a big footprint abroad. He clearly is the best debater among the candidates, which is likewise one of Mr Obama's strengths.
Indeed, the super Political Action Committee (PAC) supporting Mr Gingrich, Winning Our Future, indicated it was ready to run advertisements in Florida arguing, among other things, that Mr Obama would be able to eviscerate Mr Romney in debates.
Super PACs are ostensibly unaffiliated political action committees that can spend unlimited sums promoting any candidate or issue they want.
In a sign of how the momentum has changed, Mr Gingrich devoted the bulk of his victory speech on Saturday to attacking Mr Obama.
"If Barack Obama can get re-elected after this disaster," Mr Gingrich told jubilant supporters, "just think how radical he would be in a second term."
By contrast, Mr Romney focused on Mr Gingrich. Mr Romney said he had expected a "frontal assault on free enterprise" from Democrats, but not from fellow Republicans.
Taking aim at Mr Gingrich, he said Republicans "can't be led to victory by someone who also has never led a business or a state."
Mr Romney still has a lot of money and the biggest campaign staff, which he will need for what is now going to be a long and bruising campaign likely to run through the summer to the national convention in Florida rather than effectively end in with the looming Florida primary.
But though Mr Gingrich had "landed some blows" on Mr Romney, Jeffrey Weiss, a political consultant in Washington, said "ultimately there is no change in the game. In the end, it's still Romney's nomination."