WASHINGTON — Senators Obama and McCain are setting up a battle over who is the more proven party “maverick,” a debate that could loom large in a potential general election matchup likely to hinge on the choice of independent voters.

Republicans are already questioning Mr. Obama’s record of bipartisanship by citing a history of liberal Senate votes, and the front-runner for the Democratic presidential nomination tried to fight back yesterday, pointing to his support for less regulation, tort reform, and charter schools as areas where he has broken with Democratic Party orthodoxy.

“I think there are a whole host of areas where Republicans in some cases may have a better idea,” the Illinois senator said in an interview on “Fox News Sunday” broadcast yesterday.

Asked to name specific issues where Republicans have had better ideas, he praised the party’s efforts in the 1960s and ’70s to strip away industry regulations that he characterized as “top-down command and control.” He also highlighted education, where he noted that his support for expanded charter schools has met with opposition from the teachers’ unions. And he cited his vote for a tort reform bill in 2005 that angered trial lawyers, a group known for its generous support of Democratic candidates.

Yet the interview also underscored the challenge Mr. Obama faces as he tries to square his message of post-partisanship with a voting record that appears largely in lockstep with Democratic positions and led the Washington-based National Journal to rate him the Senate’s most liberal lawmaker.

Even as he touted his support for charter schools and “experimenting with different ways of compensating teachers,” Mr. Obama wavered on the concept of merit pay based on a standardized test score, which he said was a “big mistake.” In other areas, he described his views as more nuanced than his votes may reflect.

He said he had a “strong” belief that the state can “properly restrict” late-term abortion, but he voted against a bill that would ban the so-called partial birth procedure, citing its lack of a provision to protect the health of the mother. On judicial nominations, he pointed out that he had defended lawmakers who voted to confirm Chief Justice Roberts. Ultimately, however, Mr. Obama voted against confirmation.

“There is no record of Barack Obama performing in an effective bipartisan manner for the American people,” a spokesman for the McCain campaign, Tucker Bounds, said. “The record that he does have is pretty firmly partisan.”

Mr. Obama acknowledged that his voting record is hardly centrist, but he blamed part of that record on a political culture that thrives on wedge issues.

“It is true that when you look at some of the votes that I’ve taken in the Senate that I’m on the Democratic side of these votes, but part of the reason is because the way these issues are designed is to polarize,” he said. “They’re intentionally designed to polarize.”

The Republican strategy to cast doubt on Mr. Obama’s message mirrors the efforts Democrats have made for months to undercut Mr. McCain’s image as a straight-talking maverick. They have hammered the presumptive Republican nominee for supporting an extension of tax cuts he originally opposed as well as for his campaign’s withdrawal from the public financing system for the Republican primary, a move Democrats say is at odds with his backing of campaign finance reform earlier in the decade.

A spokesman for the Democratic National Committee, Damien LaVera, said that on issues Mr. McCain once used to burnish his independent credentials, “he’s put his politics ahead of his principles and walked away from reforms he once championed.”

Mr. Obama in particular has attacked the Arizona senator for his newfound support for extending the Bush tax cuts, saying yesterday that his “conscience took flight.”

In the Fox News interview, Mr. Obama also said he would vote to confirm the nomination of General David Petraeus as head of the military’s Central Command. He said he would listen to General Petraeus’s advice while in office, but he is also sticking to his commitment to begin withdrawing American troops quickly from Iraq, a move the general has warned against.

For both Mr. Obama and Mr. McCain, the emphasis on issues where they have broken ranks with their parties is likely to grow if Mr. Obama wins the Democratic nomination.

“It is clear that both candidates will attempt to position themselves in the center and aim at swing voters,” a political science professor at Hunter College, Andrew Polsky, said.