A dispute over a proposed gas tax holiday continued to drive Hillary Clinton and Barack Obama apart Saturday, even after the two candidates attempted to assure jittery party leaders the night before that Democrats will be united once the nominee is decided.

Entering the homestretch before the Indiana and North Carolina primaries Tuesday, the candidates took each other to task on what appears to be the hot-button issue at this stage in the contest.

Clinton is joining John McCain in calling for a summer suspension of the 18.4 cents-a-gallon federal gas tax, while Obama says it's a political stunt that would have little impact on taxpayers.

"There's not an expert out there that believes that this is going to work," Obama said of the proposal Saturday in Indianapolis. "There's not an editorial out there that has said this is actually the answer to high gas prices. ... Senator Clinton had to send out a surrogate to speak on behalf of this plan, and all she could find was -- get this -- a lobbyist for Shell Oil to explain how this is going to be good for consumers. It's a Shell game, literally."

He was alluding to the fact that Clinton supporter Steve Elmendorf, a Shell lobbyist, praised her proposal in a recent interview.

Obama characterized Clinton as a symbol of tired Washington politics, and of "phony ideas, calculated to win elections instead of actually solving problems."

Clinton spokesman Phil Singer fired back.

"Senator Obama wants Americans to pay the gas tax but Senator Clinton thinks the big oil companies should pay it this summer," Singer said.

The campaign is already running an ad attacking Obama for his opposition to a gas tax holiday, and Clinton decried his resistance Saturday in Wake Forest, N.C.

"My opponent is running ads and holding press conferences attacking my plan to try to give you some kind of break this summer," she said. "There is no contradiction between trying to provide immediate relief and having a long-term vision and a plan for what we must do to lessen our dependence on foreign oil."

Meanwhile, both candidates told the crowd Friday night at the North Carolina Democratic Party's Jefferson-Jackson Dinner in Raleigh that they would support the party's eventual presidential nominee in November.

"If Senator Obama is the nominee you'd better believe I'll work my heart out for him," Clinton said Friday, drawing cheers from a large pool of his supporters.

"I know that there are those Democrats, not just in North Carolina but across the country, that are a little worried," Obama said. "Let me say this -- we are going to be united in the fall. This party will come together. If Hillary Clinton were the nominee, I would support her in a heartbeat."

That may sound like a given, but they were words some Democrats needed to hear. If Obama doesn't score a blowout in the Tuesday primaries, Clinton has given every indication she'll take her underdog bid to the end of the primary calendar in June, leaving little time to mount a new campaign against McCain, the presumptive GOP nominee.

Clinton has been catching up to Obama in the polls in North Carolina, which offers 115 delegates. Attempting to shatter conventional wisdom her campaign is looking to pose a challenge to Obama in the state, campaigning hard in rural areas to compete against his presumed appeal in more urban areas.

"Now, you know, three weeks ago we were 39 points behind. And those were the best polls," Clinton said in Raleigh, exaggerating her come from behind. Most polls three weeks ago showed her between 10 and 20 percentage points down. Now she's down about 7 points.

"But we're working hard, we're traveling across this state. I've lost track of how many towns we've been to," she added.

Clinton has also taken the lead in polls in Indiana, which offers 72 delegates. She planned to hit both states Saturday, while Obama hunkered down in Indiana.

Obama said he still thinks his chances of taking the nomination are "terrific."

Both Democrats also showered praise Friday on former North Carolina Sen. John Edwards, who dropped out of the presidential race earlier in the year.

Edwards was not in attendance and has not endorsed either candidate, but Clinton and Obama wanted North Carolinians to know they were thinking of him.

Clinton called him "a great fighter" who ran "with compassion and conviction." She said she would "see to the finish" his fight to end poverty.

Obama said Edwards and his wife Elizabeth "set the tone for this presidential race with their courage, with their ideas, with their passion and their commitment to working people."

FOX News' Major Garrett, Aaron Bruns and Bonney Kapp and The Associated Press contributed to this report.