DURHAM, N.C. — Dueling over gas prices, Hillary Rodham Clinton and Barack Obama strained for every last vote on Monday, the eve of the Indiana and North Carolina primaries that are the biggest prizes left in their epic Democratic nomination fight.

Her TV ads promoted her plan for a summer-long gas-tax holiday and contended she was the candidate who "gets it." He said the plan was just another Washington stunt.

A combined 187 delegates are at stake in the two states, nearly half of the pledged delegates left with eight primaries to go before voting ends in a month.

Obama was the favorite in North Carolina, but both candidates campaigned vigorously there with polls showing a tightening race since Clinton's win in Pennsylvania two weeks ago. Indiana was considered tighter, with most polls in the final days showing Clinton taking the lead.

Obama hurried back and forth between the two states, pleading for votes. "I want your vote. I want it badly," he said on a factory floor in Durham, one of many stops aimed at winning over working-class voters. He is hoping to gain support from a group that has not greeted his candidacy enthusiastically -- white, mostly male construction and factory workers.

Clinton, also campaigning in North Carolina, campaigned for blue-collar votes, too, talking about the hard times the country faces.

"It's time to quit wringing our hands and start rolling up our sleeves," she said.

Pain at the gas pump has become a big issue in the long campaign that started out focusing on the Iraq war.

Oil futures reached a record of more than $120 a barrel Monday, raising concerns about even higher prices for gasoline. In a new 30-second ad featuring drivers complaining about the price of filling up, Clinton touted her plan to cut gas taxes over the summer and said Obama was just attacking her idea "because he doesn't have one."

"Barack Obama want you to keep paying, $8 billion in all," an announcer says. "Hillary is the one who gets it."

Obama responded with his own spot that said Clinton was offering "more of the same old negative politics." It points out a New York Times editorial that said she's taking "the low road" and that her criticism does "nothing but harm."

The ad didn't point out that the same editorial said Obama is contributing to the negative nature of the campaign by "increasingly rising to Mrs. Clinton's bait, undercutting his own claims that he is offering a higher, more inclusive form of politics."

Obama said the proposal to suspend the 18.4 cents-a-gallon gasoline tax and the 24.4-cent diesel tax from Memorial Day to Labor Day would provide little in actual savings to motorists. He said oil companies would quickly raise prices to make up the difference.

"It's a stunt. It's what Washington does," Obama said in Evansville, Ind.

Obama's stance was backed up by 230 economists who released a letter Monday opposing the gas tax holiday. The signers included four Nobel Prize winners and economic advisers to presidents of both parties.

A CNN-Opinion Research Corp. poll released Monday found six in 10 saying gas prices have caused financial hardship for their families. Eight in 10 said they consider it likely they'll be paying $4 a gallon sometime this year, and nearly half said they expect prices to hit $5 per gallon.

Any belt-tightening didn't extend to the presidential campaigns, with Obama outspending Clinton in both states. By Clinton campaign estimates, Obama has spent $5.6 million in Indiana to Clinton's $3.2 million. In North Carolina, the Clinton campaign said, Obama has spent $4.9 million to Clinton's $3.5 million.