"It's a Shell game. Literally," Obama said to laughter from his campaign audience, adding it would mean little for hard-pressed consumers.
The Democratic presidential rivals highlighted their differences in ads and speeches across North Carolina and Indiana, two states with primaries Tuesday.
Polls point toward a particularly close finish in Indiana, which is next door to Obama's home state of Illinois.
Surveys show him with a dwindling advantage in North Carolina, and Clinton decided to spend all of Friday and Saturday in the state before returning to Indiana for a final push.
The two primaries have 187 national convention delegates at stake.
Obama, the front-runner, leads in the overall delegate competition, 1,736.05-1602.5. Clinton won a decisive victory last week in Pennsylvania and is counting on a strong run through the late primaries to persuade convention superdelegates to help her overtake her rival.
Jolted by Thursday's defection of Joe Andrew, a former national party chairman, Clinton responded with a letter from seven other former party heads and the family of an eighth.
"Her base of support includes women, Hispanics, seniors, Catholics, middle and low-income Americans, and rural, suburban and urban voters. That's a formidable coalition tailor-made for victory in a November general election," they wrote.
They added that if the election were held today, Clinton would defeat Republican Sen.and win the White House. "Obama would lose to the presumptive GOP nominee," they wrote.
Polls are equivocal on that point. Moreover, they have been particularly volatile in recent weeks as campaign criticism takes its toll on the two Democrats and Obama grapples with controversy stemming from the rhetoric of his former pastor, the Rev. Jeremiah Wright.
Despite a fierce, occasionally personal campaign, to a surprising degree the former first lady and Obama have generally agreed on most policy issues.
That made the proposed suspension in the gasoline tax an exception.
And while there is little support among the Democratic congressional leadership for the plan, it was a disagreement that both presidential contenders appeared content to perpetuate.
"All I hear about is gas prices. Gas and diesel, everywhere," Clinton said in Kinston, N.C. "Some people say we don't need to get a gas tax holiday at all, it's a gimmick ... I want the Congress to stand up and vote. Are they for the oil companies, or are they for you?"
Later, in Hendersonville, she added, "I know where I stand and I know where my opponents stand. ... Senator Obama doesn't want us to take down the gas tax this summer and Senator McCain wants us to, but he doesn't want to pay for it."