WASHINGTON  As angry truckers encircled the Capitol in a horn-blaring caravan and consumers across the country agonized over $60 fill-ups, the issue of high fuel prices flared on the campaign trail on Monday, sharply dividing the two Democratic candidates.

Senator Hillary Rodham Clinton lined up with Senator John McCain, the presumptive Republican nominee for president, in endorsing a plan to suspend the federal excise tax on gasoline, 18.4 cents a gallon, for the summer travel season. But Senator Barack Obama, Mrs. Clinton’s Democratic rival, spoke out firmly against the proposal, saying it would save consumers little and do nothing to curtail oil consumption and imports.

While Mr. Obama’s view is shared by environmentalists and many independent energy analysts, his position allowed Mrs. Clinton to draw a contrast with her opponent in appealing to the hard-hit middle-class families and older Americans who have proven to be the bedrock of her support. She has accused Mr. Obama of being out of touch with ordinary Americans who are struggling to meet their mortgages and gas up their cars and trucks.

Mrs. Clinton said at a rally on Monday morning in Graham, N.C., that she would introduce legislation to impose a windfall-profits tax on oil companies and use the revenue to suspend the gasoline tax temporarily.