The two leading U.S. Republican presidential contenders attacked each other's credentials Saturday as they campaigned for votes in key party elections next week.

One-time venture capitalist Mitt Romney told a gathering in Michigan that he would be a “president of principle” if he can oust President Barack Obama, a Democrat, in November's national election. Romney criticized former U.S. Senator Rick Santorum , currently his main opponent for the Republican nomination, for admitting he once voted for a Republican-backed education plan even though he disagreed with it.

Romney said such unwavering political allegiance is unacceptable.

Santorum spoke at a rally of Tea Party conservative activists elsewhere in Michigan. He attacked Romney for sponsoring legislation when he was governor of Massachusetts that required the state's residents to buy health-care insurance, or pay a fine if they did not.

The law Romney sponsored became the model for national health-care reforms supported by Mr. Obama and approved by Congress – over strong opposition from Republicans including both Santorum and Romney. Santorum said he only supports health care that individuals control.

In the lengthy Republican nominating process, Romney and Santorum face key votes in two states on Tuesday – in Michigan, the hub of the U.S. auto industry, and in the southwestern state of Arizona. Opinion polls indicate Romney has pulled ahead of Santorum in the Michigan and Arizona contests.

Nationwide, public-opinion surveys show that at present, Republican voters currently are nearly evenly split in their preferences for either Romney or Santorum. Two other Republicans – former House Speaker Newt Gingrich and House member Ron Paul from Texas – are well behind the two front-runners.

In current snapshots of hypothetical November matchups, the surveys show Mr. Obama defeating both Romney and Santorum.