By THOMAS BEAUMONT and KASIE HUNT
FLINT, Mich. (AP) — Republican Mitt Romney fought Saturday to prove he is the strongest challenger to President Barack Obama, an increasingly difficult task given the tight race in his native state of Michigan against surging conservative Rick Santorum.
In the final weekend of campaigning before Tuesday's Michigan and Arizona primaries, Romney focused on central and southeast Michigan's urban and industrial centers in hopes of pulling ahead of Santorum.
With a Michigan victory, Santorum could solidify his place as a real threat to Romney heading into Super Tuesday, the 10-state sweepstakes on March. Santorum's victories so far have come in lower-turnout party caucuses.
While Romney kept most of his attention on the Democratic incumbent, he also worked to lay doubt about the core principles of his lightly funded main GOP rival.
Romney is the one facing stubborn doubts from some conservatives for his changed positions on social issues, but he tried to portray Santorum, a former Pennsylvania senator, as a Washington insider with cracks in his own conservative credentials. Santorum called such criticism "laughable" and said Michigan, where Romney was born and raised and his father was governor, was winnable.
A crowd in Lansing heard Romney accuse Santorum of caving to party leaders on issues he opposed, including financing Planned Parenthood.
"This is not time for lifelong pols who explain why they voted for this or that based on what they were asked to do by their fellow colleagues," Romney told about 300 activists gathered for breakfast at a country club. "I will be a president of principle."
Romney tried to undermine Santorum's profile as an abortion opponent by noting Santorum's backing in 1996 of fellow Pennsylvanian Arlen Specter in the GOP presidential race. "He supported the pro-choice candidate," Romney told more than 2,000 at a forum in Troy put on by a tea party umbrella group. Santorum spoke to the group, Americans for Prosperity, earlier Saturday.
Santorum, who has portrayed himself as a loyal conservative and is popular among evangelical conservatives, ridiculed Romney's claims.
"It is absolutely laughable to have a liberal governor of Massachusetts suggest that I am not a conservative," Santorum said to cheers to the same group. "He repeatedly gets up and says all these things that he didn't do that he did do. Folks, this is an issue of trust."
The volleys over principle and loyalty punctuate the all-out two-man in Michigan, leaving behind the two others in the field. Both candidates are spending heavily on television advertising, although the better-funded Romney was laying out more.
Texas Rep. Ron Paul is hardly a factor in Michigan, but is airing advertisements criticizing Santorum, an aid to Romney. Former House Speaker Newt Gingrich was nowhere to be found in the state, and has spent scant time in Arizona, which also holds its primary Tuesday.
Gingrich has acknowledged that he has no shot in Michigan or Arizona, and has predicted Romney will win. Gingrich aides argue he stands to gain by Santorum or Romney coming out of Tuesday weaker.
Gingrich is betting heavily on Georgia, the state he represented in Congress, and a strong showing in Tennessee on March 6.
Romney campaigned across southern central and southeast Michigan, where his family name is familiar, and he reminded audiences of his ties to the state.
In Lansing, the capital, Romney recalled his father's chilly winter inaugurals. Romney's wife Ann, also born in Michigan, reminisced in introducing the candidate in Troy about growing up a Tigers baseball fan and working for her father's business in the Detroit suburb.
Polls show a dead heat between Romney and Santorum, who is playing up his family's blue-collar background as the grandson of a Pennsylvania coal-miner. "This race is close. This race is winnable. But you've got to want it," Santorum told tea party members in St. Clair Shores.
He made a quick detour later to Tennessee, a Super Tuesday state that's gotten much less attention, to speak at a tea party rally in a large church in Chattanooga.
Romney's attacks are a potential problem for Santorum because he's based his candidacy on presenting himself as an uncompromising conservative, contrasting himself with Romney. The former Massachusetts governor has struggled at times to explain why he's changed his position on abortion and other issues.
In Tennessee, Santorum compared the health care bill Romney signed in Massachusetts in 2006 with the one Obama signed in 2010. The federal program is wildly unpopular with conservatives.
"Are you going to vote for someone that says one thing one day anything else the next day that's necessary to win? Or are you going to vote for someone you trust?" Santorum asked the crowd in Troy.
— — — Associated Press writer Brian Bakst in Burlingame, Calif., and Charles Babington in Chattanooga, Tenn., contributed to this report. (Copyright 2012 by The Associated Press. All Rights Reserved.)