(CNN) -- Top GOP rivals Mitt Romney and Rick Santorum campaigned in Michigan on Saturday, blasting each others' backgrounds and burnishing their own conservative credentials.
The two were in Romney's home state ahead of Tuesday's primary, which is shaping up as a critical contest. Romney, who was born in Michigan and whose father served as governor there, later moved to Massachusetts.
Speaking at the Americans for Prosperity forum in Troy, Michigan, Romney touted his "principled conservative leadership" and detailed his record opposing abortion rights as Massachusetts governor -- a record that has long been scrutinized by some social conservatives.
"I can attest to my conservative credentials by quoting someone who endorsed me in 2008," Romney said in clear reference to Santorum.
"Sen. Santorum was kind enough to say on the Laura Ingraham show, he said, 'Mitt Romney, this is the guy who is really conservative and who we can trust,'" Romney said. "And when he came out and endorsed me he said these words: 'He said he is the clear, conservative candidate.'"
Santorum has said he chose Romney in 2008 in the hopes of defeating Sen. John McCain's path to the nomination.
The former Pennsylvania senator addressed a crowd in St. Clair Shores earlier in the day, ripping into Romney for his role in implementing a state-wide health care plan as governor of Massachusetts that resembles the Affordable Care Act, dubbed by Republicans "Obamacare."
"We have one candidate that opposed government-run health care, one candidate that opposed the hoax and politicization of science," Santorum said. "We have another candidate that bought it. Not only bought it but did things as governor of Massachusetts. It's laughable for Gov. Romney to suggest that I am not a conservative. It is absolutely laughable for a liberal governor of Massachusetts to suggest that I am not the conservative in this race."
Santorum urged voters to think hard on whether or not they could trust Romney to stick to conservative ideals when the race turns to a competition against President Barack Obama.
"Folks, this is an issue of trust," he said.
After speaking in Michigan, Santorum traveled to Tennessee, where he brought a crowd to its feet and spoke about his love for the tea party.
A Romney loss in his childhood home state of Michigan 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 him as the main conservative challenger to Romney heading into the March 6 Super Tuesday primaries and caucuses.
The two Republican rivals spoke on the same day a poll of Arizona Republican primary voters showed Romney with a slight advantage over Santorum, though the gap between the candidates is shrinking.
The survey from the American Research Group showed Romney with the backing of 39% of likely primary voters, compared to 35% who support Santorum. The poll was taken February 23-24, after CNN's Arizona presidential debate, which took place Wednesday. Arizona voters go to the polls Tuesday.
The gap between Santorum and Romney narrowed since the last time ARG polled Arizona voters. In a survey taken February 13-14, Romney led Santorum 38%-31%.
But a new national poll showed that Santorum's lead has disappeared. The survey taken February 22-24 showed Santorum with the backing of 31% of registered Republicans and Republican-leaning independents, down two points from Friday and down five points from his peak on Wednesday.
Support for Romney rose to 30%, a much-needed boost for the candidate who stood at 25% on Wednesday.
For the ongoing poll, Gallup surveys more than 1,000 registered Republican voters on a daily basis. The poll drops the oldest of five days each day and adds results from the most recent day of polling.
The two other candidates in the race, former House Speaker Newt Gingrich and Texas Rep. Ron Paul, were both well behind the two front-runners. Gingrich stood at 16% and Paul was at 11%. Neither Gingrich nor Paul has seen a change in his numbers since Thursday.
CNN's Kevin Liptak and Rachel Streitfeld contributed to this report.