In Arizona, Romney has long been the clear favorite, but his supporters say Santorum has picked up support since his surprise victories in Feb. 7 contests in Colorado, Minnesota and Missouri. A CNN-Time magazine poll released Tuesday showed that Romney’s lead is four percentage points. The fact that voting has been underway all month is expected to boost Romney’s chances.

Romney won Michigan during his 2008 presidential campaign, though he lost Arizona to that state’s Sen. John McCain. Losses on Tuesday, particularly in his home state, would shake the foundation of his campaign and send shudders through a Republican establishment increasingly worried about the party’s prospects of defeating President Obama in November.

Santorum’s surge has raised fresh questions about why Romney has been unable to consolidate GOP support, and it has spurred talk about whether it is too late for another candidate to enter the race. It also has sparked new discussion about whether the race could be decided on the floor of the Republican National Convention in August in Tampa.

But the sudden rise of Santorum, a former senator from Pennsylvania, has brought critical attention to his record. On Tuesday, the Huffington Post reported that his strong antiabortion position was at odds with his earlier views. The Web site found a Philadelphia Magazine article from 1995 in which Santorum said: “I was basically pro-choice all my life, until I ran for Congress.” According to that article, Santorum said he changed his view after reading scientific literature about the practice. Late Tuesday, Santorum adviser John Brabender, after checking with the candidate, said, “He never held a pro-choice position.”

Campaigning in Michigan on Tuesday, Romney hammered Santorum for a second straight day, but not on social issues. Instead, the former governor accused Santorum of being a Washington insider who never left the capital after losing his Senate seat in 2006.

Romney said his rival had supported more spending and debt and voted for numerous earmarks. “I don’t think these are consistent with the principles of conservatism,” Romney said in Shelby Township. “I don’t think Rick Santorum’s record is that of a fiscal conservative.”

Another contender, Rep. Ron Paul (Tex.), piggybacked on Romney’s attacks with a television ad that excoriates Santorum on spending, calling him a “fake” fiscal conservative.