An aggressive Romney repeatedly put Santorum on the defensive in a CNN debate on Wednesday and attacked the former U.S. senator and staunch social conservative for supporting big-spending government programs.
Romney has also battled his way into a slight lead in a new poll in Michigan, which along with Arizona will hold a primary contest on February 28. Romney had trailed Santorum by as much as double digits a week ago in the Michigan race.
The pace of the Republican race quickens dramatically next month, with 22 state nominating contests in March, including 10 on "Super Tuesday" March 6.
"This debate will really give Romney some momentum heading into the Michigan primary," Republican strategist Ron Bonjean said. "Santorum clearly was having a hard time having to apologize and explain so many of his votes."
The debate was Santorum's first time in the political spotlight since charging into a national lead after winning three contests on February 7 in Missouri, Minnesota and Colorado.
But his sometimes awkward defense of votes he cast in the Senate, including his claim that politics was a "team sport" and that he sometimes had to vote for bills that did not like, could hurt his support among conservatives tired of Washington politics-as-usual.
"There is always an excuse," rival Ron Paul said of Santorum's various explanations. "That's what the problem is with Washington," said Paul, a Texas congressman who described Santorum's fiscal conservatism as "fake."
The primary in Michigan, where Romney was born and raised and his father was an auto executive and popular governor, has become a must-win for the former Massachusetts governor and former head of a private equity group.
A Romney loss there would be crippling blow that would set off alarm bells among Republicans about his ability to win the allegiance of conservatives, ensuring a long and potentially draining battle for the nomination.
But a win in Michigan and Arizona would put Romney back in command in a race that has featured a series of conservative rivals who have risen to challenge him only to fall back into the pack.
Santorum, a former U.S. senator from Pennsylvania, is the latest candidate to seriously contest the party's nomination to face Democratic President Barack Obama in a November 6 general election.
On Thursday, Romney will speak to a builders' group in Arizona before heading to Michigan for an evening rally of the conservative Tea Party movement. On Friday, he will address the Detroit Economic Club. Santorum is fund-raising in Texas on Thursday.
POLLS TIGHTEN
New polls in Michigan and Arizona on Wednesday showed Romney gaining ground on Santorum. He held a slight 2-point edge on Santorum in an NBC/Marist poll in Michigan, and a 16-point edge in an NBC/Marist poll in Arizona, where a CNN/Time poll earlier in the week gave him just a 4-point edge.
The broader campaign battleground in March could allow Romney to exercise his financial and organizational edge on his rivals, and put him in a strong position to knock them out and wrap up the race at least by April, if not earlier.
Santorum may have missed his chance at Wednesday's debate.
"Santorum needed a strong performance in the debate and he didn't get it," Republican strategist Ford O'Connell said. "All eyes were on him. He had an opening and he missed it."
A devout Catholic, Santorum is best known for conservative positions on religious freedoms, abortion and gay marriage, but Romney took aim at him frequently on Wednesday night as a big spender.
"I'm looking at his historic record - voting for raising the debt ceiling five different times without voting for compensating cuts," Romney said.
"Voting to fund Planned Parenthood, to expand the Department of Education. During his term in the Senate, spending grew by some 80 percent of the federal government," he said. Planned Parenthood provides abortion, birth control and other health services to women.
Former U.S. House of Representatives Speaker Newt Gingrich, who represented Georgia while he served in Congress, is hoping to rebound from his recent drop in the polls to reassert himself during some of the March contests in Southern states like Georgia, Oklahoma, Mississippi, Alabama and Louisiana.
No other debates are scheduled in the Republican race, although more could be added if the contest stretches into April and beyond.
(Writing by John Whitesides; Editing by Alistair Bell and Paul Simao)
Stephen Crowley/The New York TimesProtesters in Orange County, Calif., during President Obama’s visit there. Republicans criticized the president for combining campaign stops with official events. 
SAN FRANCISCO — If there were any doubts that President Obama had plunged fully into his re-election campaign, they were erased on Thursday as the president spent an entire day hopscotching some of California’s fanciest neighborhoods in search of dollars to finance his bid for a second term. 
A one-stop destination for the latest political news — from The Times and other top sources. Plus opinion, polls, campaign data and video.
President Obama spent much of Thursday at fund-raisers in San Francisco, with a stop at a restaurant in Chinatown. In political circles a fund-raising trip of this magnitude is called a “money run,” and advisers say that by the time the last wineglasses will be clinked — on Friday, at a fund-raiser at the Westin in Bellevue, Wash. — Team Obama will have raised around $8 million. While Mr. Obama is mixing in a couple of public stops — he visited a manufacturing company in Milwaukee on Wednesday and will stop at a Boeing plant on Friday — the mostly political nature of his trip produced Republican grousing that Mr. Obama was doing “what he does best,” in the words of the Republican National Committee’s spokeswoman, Kirsten Kukowski: “Campaign in Hollywood with the rich and famous who will open their pocketbooks for his campaign coffers.” The White House press secretary, Jay Carney, shrugged off the Republican criticism, saying, “We’re consolidating a lot of events out here because he’s here, and then there will be periods of relatively few political events, and then more concentration, depending on what’s on the schedule.” As to the criticism that taxpayers are paying for part of what is a fund-raising trip because the campaign stops are combined with official events, Mr. Obama’s advisers noted that has long been the case during presidential re-election seasons. “We do it absolutely by the book and in the same manner that President Bush did and President Clinton did,” Mr. Carney told reporters traveling aboard Air Force One. Mr. Obama is trying to use the lengthy Republican presidential nomination fight to shore up Democratic financing for the election year. Just two weeks ago, the Obama campaign, in a turnaround, began officially encouraging supporters to donate to a Democratic “super PAC,” Priorities USA. Obama campaign officials characterized their decision as a necessary one meant to combat similar Republican political action committees, particularly those set up to back Mitt Romney. At the various events on Wednesday and Thursday, Mr. Obama mostly stuck to the same script, telling supporters at one fund-raiser, co-hosted by the actor Will Ferrell: “Part of our job is to say, as tough as it is, as incremental as it sometimes seems, things have changed. And they can change more if we fight for it, if we’re determined, if we have confidence in each other, if we decide to unite instead of divide.” Mr. Obama referred to signs of an uptick in the economy, including jobs growth and a revival of the auto industry after the bailout that he backed. And because this was, after all, a group as solidly Democrat as any the president is likely to encounter, he even talked about the health care bill he pushed to passage. Mr. Obama’s arrival on Thursday in San Francisco coincided with his campaign’s announcement that it had opened its first office dedicated solely to high-tech work in the race, like setting up Twitter feeds, Facebook pages and other Internet applications. Campaign aides called it a “technology field office,” and said they believed it to be a first for a presidential campaign. Katie Hogan, deputy press secretary with the Obama campaign, said that one of the office’s missions would be to serve as a place for Web-savvy volunteers to come with ideas about how to increase the campaign’s reach through the use of the Internet.