Democrats in Pennsylvania make their choice on Tuesday in the presidential nominating race between Hillary Clinton and Barack Obama, with Clinton aiming for a big win to keep her flickering White House hopes alive.

The New York senator is favoured in Pennsylvania, but needs a convincing victory to gain ground on Obama in the Democratic race and convince party leaders she is the best candidate to face Republican John McCain in November's presidential election.

Voting in Pennsylvania, the first state to cast ballots in six weeks, closes at 8 pm EDT and the first results will be available soon afterward.

Clinton's one-time 20-point lead has slipped to single digits in many polls amid an onslaught of advertisements by Obama, who has heavily outspent her in the state. But both camps tried to play down expectations ahead of the vote.

"I'm not predicting a win. I'm predicting it's going to be close and that we are going to do a lot better than people expect," Obama told a Pittsburgh radio station on Monday.

The two candidates spent the final day of an increasingly sharp fight scouring the state for last-minute support. Clinton released an ad featuring images of al Qaeda leader Osama bin Laden and touting her strength.

"You need to be ready for anything -- especially now, with two wars, oil prices skyrocketing and an economy in crisis," the ad's narrator says. "Who do you think has what it takes?"

Obama's camp accused her of using "the politics of fear" and he released his own ad in response. "Who in times of challenge will unite us -- not use fear and calculation to divide us?" the ad asked. 

The Pennsylvania vote opens the final phase of Clinton and Obama's hard-fought duel for the nomination. Nine more contests are scheduled before the campaign concludes on June 3.

Obama has a nearly insurmountable lead in popular votes won during the first three months of the primary battle and in delegates who will choose the nominee at the August convention. But neither can clinch the nomination without the help of superdelegates -- nearly 800 party insiders who are free to support either candidate.

Clinton hopes a big win in Pennsylvania ignites a strong run through the last nine contests, convincing superdelegates she is the candidate who can capture the big states that will be crucial in November.

A narrow Clinton win would probably keep her in the race, but not stem another round of calls among Democrats for her to step aside and let Obama focus on the race with McCain.

"Someone's going to be the nominee and I think that someone is going to be Barack Obama. But it will take some time," Obama strategist David Axelrod told reporters.

In contrast, an Obama win would almost certainly increase pressure on Clinton to drop out of the race and ignite a move by superdelegates toward the Illinois senator.

The two candidates spent the weeks before the vote battling campaign controversies and courting Pennsylvania's big bloc of blue-collar voters. Obama went bowling and hoisted beers with voters, while Clinton went door-to-door in working class neighborhoods.

Obama was on the defensive at times over the inflammatory comments of his former pastor and his own comments about the bitterness of residents in economically struggling small towns. - Reuters