The fate of Hillary Clinton's presidential bid rested with voters in Ohio and Texas on Tuesday, as they cast their ballots in her hard-fought Democratic duel with rival Barack Obama.

The New York senator, battling to snap Obama's string of 11 consecutive victories, needs wins in both big states to rejuvenate her struggling campaign and ensure the race moves on to Pennsylvania on April 22.

Losses in even one of the states could set off a stampede of party support for Obama, increase pressure on her to drop out and make it even tougher to cut Obama's lead in pledged delegates who choose the party nominee in November's presidential election.

Polls show the two Democrats in tight races in both Ohio and Texas. Rhode Island and Vermont vote on Tuesday as well.

"I think I know what's happening and I believe that we're going to do well tomorrow," Clinton told reporters in Ohio, dismissing suggestions of impending doom.

"Then we move on to Pennsylvania and the states still ahead. So I'm just getting warmed up."

Tuesday's contests also could put Republican front-runner John McCain, an Arizona senator, close to clinching his party's nomination. McCain is favoured to beat his last remaining major challenger, former Arkansas Gov. Mike Huckabee, in all four states.

Voting ends in Ohio at 7:30 p.m. EST (12:30 a.m. British time on Wednesday) and all voting in Texas will be over by 9 p.m. EST.

Clinton and Obama made last-minute appeals for votes in Ohio and Texas on Monday. Clinton rose before dawn to tour a car plant in Toledo, Ohio, and closed the day with a televised town hall meeting in Texas with time bought by her campaign.

She renewed her attacks on the Illinois senator over the North American Free Trade Agreement, an unpopular treaty in Ohio, which has suffered from manufacturing job losses.

Both candidates say the treaty should be renegotiated, but the Clinton campaign has highlighted reports an Obama adviser assured the Canadian government his opposition was political rhetoric and not substantive.

Obama said his adviser, Austan Goolsbee, had simply reiterated to Canadian officials his views on the treaty, and Canadian officials backed his version of the story. Earlier denials that a meeting took place, Obama said, were "the information I had at the time."

LOWER EXPECTATIONS

Clinton's campaign aides scrambled on Monday to lower the expectations for the contests, backing away from predictions she needed to win both Ohio and Texas, which have a combined 334 delegates at stake.

Obama noted the high stakes during a late rally in Houston on Monday night.

"So here we are with the possibility of winning the nomination," Obama said, adding there was a tendency in a successful campaign to feel things would always go right.

"One of the things that I've learned is that what makes this powerful is not that things always go easy, but rather that we are willing to go forward when it's hard," he said.

Like her husband, who nicknamed himself "The Comeback Kid" for his improbable rise to the White House in 1992, Hillary Clinton has dodged disaster before. In January, Obama appeared ready to deal her a knockout blow in New Hampshire after his big win in Iowa, but she defied polls and swept to victory.

A few weeks later, her landslide loss in South Carolina and a string of high-profile endorsements gave Obama momentum heading into Super Tuesday contests around the country, but she battled him to a draw and won most of the biggest prizes of the night in California, New York and New Jersey.

Under Democratic rules allowing the losers in each state to win a proportional amount of delegates, Clinton will not be able to close the gap on Obama among pledged delegates unless she scores big wins in Ohio and Texas that appear to be beyond her grasp.

A count by MSNBC gave Obama 1,194 delegates to Clinton's 1,037 heading into Tuesday's showdowns, a significant lead but well short of the 2,025 needed to win the nomination.

Obama campaign manager David Plouffe released a memo questioning Clinton's options if she does not make major strides in catching up to Obama in pledged delegates.

"While the Clintons gamely continue to try to move the goal posts, at some point there has to be a reckoning," he said. "What is their path to secure the nomination? No amount of spin can change the math."