DURHAM, N.C. - (AP) 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.

Late Monday, Clinton acknowledged the difficulty of getting such a gax-tax suspension enacted in the next few weeks. "Realistically, it's tough," Clinton told reporters on her campaign plane.

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.

Both candidates have had supporters spending money in Indiana as well. The Service Employees International Union, which is backing Obama, spent about $1.1 million in the state, much of it on ads. The American Leadership Project, which has received most of its money from labor groups backing Clinton, spent more than $1 million on ads in Indiana that questioned Obama's economic policies.

North Carolina and Indiana are important because they are the largest states left to vote, but they cannot mathematically settle the nomination. A candidate needs 2,025 delegates to win, and Obama had 1,745.5 to Clinton's 1,608 Monday.

Obama continued to close Clinton's long-held lead among superdelegates, those party leaders who aren't bound by the outcome of state contests. He picked up two from Maryland Monday, leaving him trailing Clinton 269-255.

Obama campaigned in both Indiana and North Carolina on Monday, closing the day with an outdoor rally on American Legion Mall in Indianapolis headlined by Stevie Wonder.

Wonder opened his act by doing musical scales unaccompanied, using Obama's name as he went up and down the register before encouraging the crowd of thousands to join and follow him.

"Barack Obama inspires me to write songs ... and encourages me that we can come together and be a far greater country than ever before," Wonder said. He sang a series of hits, including "Signed, Sealed, Delivered I'm Yours," one of the Obama campaign theme songs.

Clinton's main hope for winning the nomination is to persuade most of the roughly 220 superdelegates still undecided to disregard his lead in the delegate chase and support her instead. The Clinton campaign also hopes to get a boost by getting delegates from Michigan and Florida seated.

The Democratic National Committee disqualified those delegates last year because the two states held their primaries too early. Clinton won both contests after all the candidates pledged to boycott the campaigns.

The DNC's Rules and Bylaws Committee has scheduled a meeting May 31 to consider seating delegates from the two states. Asked about a report over the weekend in the Huffington Post that the Clinton campaign is encouraging supporters on the committee to reseat the delegations, Clinton spokesman Howard Wolfson said, "If it's a secret that we want the delegations from Florida and Michigan seated, it's the worst kept secret in American politics."