WASHINGTON — The debate surrounding a gas tax holiday this summer has emerged as the chief policy divide between senators Obama and Clinton heading into a pair of primaries in Indiana and North Carolina tomorrow.

Mr. Obama is accusing Mrs. Clinton of pandering to voters by supporting a temporary repeal of the tax, while the former first lady has characterized her Democratic opponent as a candidate who is out of touch with middle-class concerns and is siding with economists instead of cash-strapped families.

The candidates are airing dueling negative ads on the issue in both states, and each defended their positions yesterday in network television appearances.

For Mr. Obama, the policy fight has offered a chance to change the subject from the re-emergence of his former pastor, the Reverend Jeremiah Wright Jr., as well as an opportunity to apply his anti-Washington message to a substantive issue.

The Illinois senator yesterday called the gas tax proposal “a classic Washington gimmick,” noting that Mrs. Clinton had embraced the idea after it was first broached by the presumptive Republican nominee, Senator McCain.

“It is a political response to a serious problem that we have neglected for decades,” Mr. Obama said on NBC’s “Meet the Press.” “That’s what Washington does,” he added later. “We don’t deal with the serious issues that are in front of us. We try to figure out what’s going to poll well and what can we do to get through the next election.”

Mr. Obama has relied on support for his position from economists, editorial boards, Democratic leaders in Congress, and even several backers of Mrs. Clinton, including Governor Paterson of New York and Governor Easley of North Carolina. A gas tax holiday, he says, would save consumers less than $30 over the course of the summer, or, as he often puts it, “half a tank of gas.”

Mrs. Clinton’s proposal differs from Mr. McCain’s in that she would pay for the loss of revenue with a windfall profits tax on oil companies. Mr. Obama notes, however, that she has already proposed to use a profits tax to pay for investment in new energy sources.

The Clinton campaign has made little effort to counter Mr. Obama’s argument from a policy standpoint. Asked yesterday by ABC’s George Stephanopoulos to name a single “credible” economist who supported the gas tax break, Mrs. Clinton deflected the question. “I’ll tell you what, I’m not going to put my lot in with economists,” she said.

A senior economic adviser to Mrs. Clinton, Gene Sperling, stressed on CNN’s “Late Edition” that the gas tax holiday did not conflict with the candidate’s long-term energy agenda and that most of the windfall profits tax would go to pay for new investment.

Both campaigns have tried to use the gas tax issue to exploit their opponent’s vulnerabilities. A Clinton spokesman, Howard Wolfson, suggested to reporters on a conference call yesterday that Mr. Obama “just doesn’t seem to seem to understand that middle-class families are hurting.” A top Obama supporter, Senator McCaskill of Missouri, responded that Mrs. Clinton’s proposal was “pandering at its purest form.”

“Real people understand what this is. They smell it,” she told reporters on a separate conference call.

Mr. Obama yesterday also accused Mrs. Clinton of playing politics with foreign policy, comparing her threat to “obliterate” Iran if it launched a nuclear attack on Israel to the “bluster” and “saber rattling” of President Bush.

“It’s not the language that we need right now, and I think it’s the language of George Bush,” he said on “Meet the Press.” He added that he would “act forcefully and appropriately” in response to an attack on Israel, but he would not speculate on specifics.

The votes tomorrow will test both Mr. Obama’s stance on the gas tax suspension and on the lingering damage that Rev. Wright has inflicted on his campaign. After Rev. Wright amplified many of his more inflammatory remarks in a press tour late last month, Mr. Obama effectively severed their ties, voicing outrage and saying their relationship had changed.

Victories for Mr. Obama in both Indiana and North Carolina could give him another chance to push Mrs. Clinton out of the race. Recent polls, however, have shown momentum in her favor. She has opened up a six-point lead in Indiana, according to the Real Clear Politics average, while she has narrowed an Obama advantage in North Carolina that once was in the double digits. Recent surveys place Mr. Obama’s lead in North Carolina at between five and nine points.

While Indiana is heavy on the white, working-class Democrats that have favored Mrs. Clinton thus far, it also borders Illinois and is reached by the Chicago press and broadcast market, giving Mr. Obama a partial home-field edge. North Carolina favors Mr. Obama in large measure because of its sizable African-American population.

The Clinton campaign has worked to set a high bar for Mr. Obama, pointing out that his campaign had factored victories in both Indiana and North Carolina into one of its delegate projection spreadsheets earlier this year.