US Democrats Barack Obama and Hillary Clinton have wooed working-class voters in Indiana and North Carolina in a late push for support on the eve of critical presidential showdowns in the two states.

The senators, embroiled in a gruelling nominating struggle that has split the party, started early on a long day of campaigning that will take them to both states voting in Tuesday's contests.

Ms Clinton renewed her criticism of Mr Obama for opposing her proposal to lift the federal fuel tax for the summer, which Mr Obama and many economists have rejected as political pandering.  Ms Clinton has used the issue to emphasise her support for workers struggling with record gas prices in a faltering economy.

"Senator Obama doesn't want to do anything," Ms Clinton, a New York senator, told a rally at a community college in Greenville, North Carolina. "You don't hire a president to make speeches. You hire a president to solve problems."

Mr Obama, an Illinois senator, made an early-morning visit to a construction site in Evansville, Indiana, where he talked to workers about jobs and high gas costs. During an appearance on NBC's Today show, he said the gas tax holiday was a dishonest approach to a real problem.

"There is not a single economist or editorial that I've read that says that this is a good idea, and the reason is, is because it's not being honest with the American people," Mr Obama said. "People don't need symbolic relief, they need real relief."

Prolonged, bitter battle

Indiana and North Carolina, with a combined 187 delegates to the August nominating convention at stake, are the biggest prizes remaining in a state-by-state Democratic race. There will then be only six contests left.

Mr Obama leads Ms Clinton in the race for the 2,025 delegates needed to win the nomination to face Republican John McCain in November's presidential election.

Voting in the elections on Tuesday ends in Indiana at 7pm EDT (0900 AEST Wednesday) and in North Carolina at 7.30pm EDT, with results expected soon after.

Wins for Mr Obama in both states would effectively end Ms Clinton's chances of overtaking him in either delegates or popular votes cast in the state-by-state battle.

But a sweep of the two states by Ms Clinton would fan doubts about Mr Obama's electability and prolong a rough patch where he has been on the defensive over a big loss in Pennsylvania and his relationship with his controversial former pastor Reverend Jeremiah Wright.

Mr Obama leads in polls in North Carolina, although his once double-digit advantage over Ms Clinton has shrunk to single digits. The two are running close in Indiana, where most polls show Ms Clinton with a slight lead.

Superdelegate decision

Neither candidate will win enough delegates before the voting ends on June 3 to clinch the nomination, leaving the decision to nearly 800 superdelegates - elected officials and party insiders - who are free to back any candidate.

A split decision on Tuesday would leave the race largely unchanged, with the candidates trying to convince the superdelegates they would be the best Democrat to take on Mr McCain.

Ms Clinton, in a morning appearance on NBC's Today show, ducked questions about how she would make that case if she cannot catch Mr Obama in pledged delegates to the convention won in state contests.

"This has been a very dynamic election. It changes from week to week. But I feel very good about the case that I'm making to the American people and the response that I'm getting," she said.

Mr Obama made a morning visit to a union hall in Indiana, where he consumed a breakfast of scrambled eggs, sausages and biscuits. He spoke optimistically about his prospects in the state.

"I think our chances are good. We've had a great week of campaigning and I desperately want every single vote," he said.