Barack Obama and Hillary Clinton faced their latest day of destiny on Tuesday as Democrats in Indiana and North Carolina geared up to vote in the party's electrifying presidential race.

Opinion polls pointed to another messy draw on the biggest single day of voting left in the epic battle for the Democratic nomination, with Obama tipped to win the North Carolina primary and Clinton ahead in Indiana.

The rivals raced through a frenetic dawn-to-midnight campaign swing in the two states on Monday but both signaled the contest would drag on through the bitter end of the primary calendar on 3 June, in Montana and South Dakota.

"We hope to do as well as we can, we started out pretty far behind," Clinton told reporters on a late-night flight across Indiana.

"I try to do as best as I can, I don't make predictions," she said, just ahead of her last rally in the state.

The former first lady also took another swing at Opec, after oil prices busted the symbolic 120 dollars-a-barrel barrier.

"They can no longer be a cartel, a monopoly that get together once every couple of months in some conference room in some plush place in the world," Clinton said, sparking cheers in a fire station in Indiana's Chicago suburbs.

Clinton's camp admits she cannot overtake the Illinois senator in the count of pledged delegates who will formally anoint the nominee at the Democratic convention in August.

So she is pinning her hopes on persuading nearly 800 superdelegates, who look set to have the deciding vote, that he cannot beat Republican presidential candidate John McCain in November.

But Obama dismissed Clinton's claims he may be a general election liability, after a punishing month in April which sucked some of the euphoria out of his candidacy.

"Once you're the front-runner, then it is, I think, the obligation of the candidates who are behind to try to whack you over the head, and the press is happy to oblige," Obama said.

"So there was a kitchen-sink strategy employed that was throwing a whole bunch of stuff at me.

"But if you think about it ... the fact that we're still standing here and still moving forward towards the nomination, I think, indicates the degree to which the core message of this campaign is the right one."

Analysts say Clinton (60) needs to take the rustbelt state of Indiana to at least halt a flow of Democratic "superdelegates" to Obama and stay in the race.

Tuesday's voting was likely to shed light on whether Obama, who is vying to become America's first black president, has been damaged by the fallout from racially tinged remarks by his former pastor Jeremiah Wright.

On the final stretch of campaigning, the two rivals fought a vicious television advertising war.

"What's happened to Barack Obama?" asked a Clinton ad, focusing on his dismissal of her plan for a temporary moratorium in gasoline taxes, but also highlighting his recent troubles.

In Indiana, a rolling average of polls by RealClearPolitics.com gave Clinton a five-point lead over Obama — about 49 percent to 44. In North Carolina, which has a large black population, Obama was up seven, 50 to 43 percent.

Combined, the two states were electing 187 pledged delegates on Tuesday — 115 in North Carolina and 72 in Indiana. After Tuesday, 217 elected delegates will be up for grabs in the remaining six contests.

The day's voting was to begin in parts of Indiana at 6am (1000 GMT) and close in North Carolina at 8.30pm (0030 GMT).

RealClearPolitics gives Obama 1491 pledged delegates from all the races so far to Clinton's 1337. Neither can reach the winning line of 2025 without backing from the superdelegates, party officials free to vote either way.

AFP