By John Whitesides, Political Correspondent
WASHINGTON (Reuters) - Hillary Clinton sought a convincingwin to keep her flickering White House hopes alive on Tuesdayas Pennsylvania Democrats chose between her and Barack Obama intheir presidential nominating contest.
Polls close at 8 p.m. EDT (1 a.m. British time) withresults expected shortly thereafter.
Clinton, a New York senator, is favoured in Pennsylvaniabut needs a substantial victory to gain ground on Obama in theDemocratic nomination race and convince party leaders she isthe best candidate to face Republican John McCain in November'spresidential election.
Her one-time 20-point lead in the state has slipped tosingle digits in many polls amid an onslaught of advertisementsby Obama, who has heavily outspent her in the first nominatingcontest in six weeks. Both camps tried to play downexpectations before the vote.
"This was always an uphill climb but what we feel is thatwe've made significant progress," Obama said at a diner inPittsburgh. "A lot of it is going to depend on turnout todayand it's really hard to gauge."
Turnout was heavy at many polling places, local media said.A record number of Pennsylvanians are registered to vote in thestate's first contested Democratic primary since 1976 and onepoll showed most new registrants plan to vote for Obama.
The Pennsylvania vote opens the final phase of theDemocrats' hard-fought duel for the nomination. Nine morecontests are scheduled before the campaign ends on June 3.
PARTY LEADERS COULD DECIDE NOMINEE
Obama has a nearly insurmountable lead in popular votes wonduring the first three months of the primary battle and indelegates who will choose the nominee at the August convention.But neither can clinch the nomination without the help ofsuperdelegates -- nearly 800 party insiders who are free tosupport either candidate.
Clinton hopes a big win in Pennsylvania ignites a strongrun through the last nine contests, convincing superdelegatesshe is the candidate who can capture the big states that willbe crucial in November.
"I think maybe the question ought to be: Why can't he closethe deal? With his extraordinary financial advantage, why can'the win a state like this one, if that's the way it turns out?"Clinton told reporters in Conshohocken, a Philadelphia suburb.
A narrow Clinton win would probably keep her in the racebut could prompt calls among Democrats for her to step asideand let Obama focus on the race with McCain. Clinton hasalready rallied from the brink of elimination twice in thestate-by-state nominating process that started in January.
"I do believe we're coming to the end of this process,"Obama said in Philadelphia, citing his lead in the delegatecount and popular vote and the number of states he has won.Obama said he was still on track to win the nomination.
Clinton supporters said she should stay in the race untilthe August convention even if she loses Pennsylvania.
"I think she should go all the way to the end. I don'tthink she should drop out at all," said Anita Barton, 56, at apolling place in Conshohocken.
Clinton spent the final day of the increasingly sharpnominating fight touting her strength. She released an adfeaturing images of al Qaeda leader Osama bin Laden and warnedin an interview with ABC that, as president, she could "totallyobliterate" Iran if it attacked Israel.
Clinton's last minute ad warned voters that they need to"be ready for anything" during a time of war and worseningeconomic prospects. "Who do you think has what it takes?" thead's announcer asked.
Obama's camp released an ad in response. "Who in times ofchallenge will unite us -- not use fear and calculation todivide us?" it asked.
Obama, who would be the first black president, was on thedefensive at times because of inflammatory comments of hisformer pastor and his own comments about the bitterness ofresidents in economically struggling small towns.
Clinton, who would be the first woman president, had toapologize for fabricating a story about facing sniper fireduring a 1996 visit to Bosnia when she was first lady.
Clinton's husband, former President Bill Clinton, told aPhiladelphia radio station on Monday the Obama campaign had"played the race card" earlier in the contest but on Tuesdaysought to play down the comment.
"That's not what I said," Bill Clinton told an NBCreporter. "You always follow me around and play these littlegames and I'm not going to play your games today."
(Additional reporting by Jeff Mason, Caren Bohan, DavidMorgan and Paul Eckert; Editing by Doina Chiacu)
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(To read more about the U.S. political campaign, visitReuters "Tales from the Trail: 2008" online athttp:/blogs.reuters.com/trail08/)