(CNN) -- Republican presidential candidates Mitt Romney and Rick Santorum spent Friday trading barbs and stumping for votes in Michigan, where polls show the two candidates in a virtual tie four days before what has emerged as a critical contest.
A Romney loss in his childhood home state would undermine the former Massachusetts governor's claim to front-runner status and throw the GOP race into turmoil. A Santorum win would clearly establish the former Pennsylvania senator as the main conservative challenger to Romney heading into the March 6 Super Tuesday primaries and caucuses.
Top Romney adviser Stuart Stevens promised a Michigan win for his candidate after Wednesday night's CNN-sponsored debate.
Newt Gingrich -- who appears to have all but bypassed the February 28 Michigan and Arizona primaries -- spent Friday wrapping up a trip to Washington state, which holds its caucuses on March 3.
Ron Paul had no public events on Friday, but will spend the weekend stumping for votes in Michigan.
While Romney and his well-financed allies are presumed to be outspending Santorum in Michigan, Santorum's campaign released a new television ad Friday slamming Romney for allegedly favoring the interests of Wall Street over those of Detroit.
"He supported the Wall Street bailouts, while turning his back on Michigan workers," the narrator in the ad says. "But Rick Santorum's made in the U.S.A. plan changes everything. It cuts taxes for Michigan manufacturers to zero, so we can bring back American jobs from overseas."
Romney's team dismissed the ad, saying Santorum's campaign is "lashing out with desperate and false attacks in order to prop up his sinking campaign."
The former senator claimed in an interview Friday with CNN's Gloria Borger that it was Romney who was guilty of "attacking (him) falsely."
"What you can trust is that (Romney) is not going to tell you the truth about his record, and he's going to try to distort somebody else's record for him to suggest that he is more conservative than me and more reliable," said Santorum, before attending evening events in the Michigan communities of Walled Lake and Lincoln Park.
Santorum also hit back against criticism during Wednesday night's debate that he voted for former President George W. Bush's No Child Left Behind education reform plan despite supposed ideological objections to the measure.
"It was against the principles I believed in, but you know when you're part of the team, sometimes you take one for the team, for the leader, and I made a mistake," Santorum said at the time. "You know, politics is a team sport ... Sometimes you've got to rally together and do something."
Romney said Thursday that he wondered "which team (Santorum) was taking it for."
"My team is the American people, not the insiders in Washington, and I'll fight for the people of America, not special interests," the former governor said.
Asked by CNN on Friday what team that Romney is playing for, Santorum said, "Well, I would say that's the left. I would say that's the problem with Gov. Romney in this election."
Romney's campaign vehemently objected to claims made in a Santorum statement issued on Friday -- alleging that the then governor "passed socialized medicine that included $50 dollar abortions, bragged about not lining up with the NRA-appointed liberal activist judges to the Massachusetts bench, and was hanging out at Planned Parenthood events."
Meanwhile, Romney went to several cities in Michigan, where he grew up and where his father once served as governor.
He discussed his plan to lower marginal tax rates for individuals by 20% in a speech at Detroit's Ford Field, then headed about 25 miles northeast to hobnob with diners at a Mount Clemens restaurant called The Mitt. In the evening, he held a town hall event in Kalamazoo.
It's all part of the effort to win the Great Lakes State's hotly contested primary.
An American Research Group poll released Thursday shows 38% of likely Michigan GOP primary voters backing Santorum, and 34% supporting Romney -- right on the edge of the survey's margin of error of 4 percentage points. The poll was conducted before Wednesday's debate.
Nationally, a Gallup Daily Tracking Poll released Friday suggested that Santorum had backing from 33% of registered Republicans, followed by Romney with 27% support. That 6-percentage point polling advantage represents a drop from the 10-point lead Santorum had Tuesday.
The phone survey of 1,187 Republicans and Republican-leaning independents was conducted between February 19 and February 23, and has a sampling error of plus or minus three percentage points.
In the same poll, Gingrich's backing stood at 16% and Paul's was at 11%.
The former House speaker, in an interview Friday with CNN's Piers Morgan, aimed his fire primarily at President Barack Obama -- for the government's apology for the burning of Qurans at a military base in Afghanistan, the federal bailout of automakers General Motors and Chrysler and the administration's energy policies, among other issues.
As to his own campaign's strategy, Gingrich said he is focused on the 10 contests slated for March 6, or Super Tuesday, over the upcoming ones in Arizona and Michigan. He pointed out how many contenders have soared and fallen since the GOP race began, suggesting that more surprises could be in store.
"If you think this race looks wild from your side, you ought to be with us," he said. "(His wife) Callista and I feel like we've been on a roller coaster -- sort of like Space Mountain in Disney where it's all dark and you don't know where you've been, where you're going, or where you are."
CNN's Alan Silverleib and Gabriella Schwarz contributed to this report.