* Hillary Clinton tipped to win Pennsylvania primary
* But even with a win she will still be behind
* Pennsylvania primary: Latest campaign pictures
HILLARY Clinton needs a big win in today's Pennsylvania primary to keep alive her improbable US presidential campaign.
The New York senator was tipped for victory in late opinion polls, but many observers think it will take a double-digit triumph to stave off more calls for her to quit the epic Democratic nomination race.
Senator Clinton, however, said that a simple victory would be enough. "I think a win under any circumstances is a terrific accomplishment," she said after greeting supporters at a polling station who chanted "Madam President".
Polls have closed in the state and US media have reported a tight battle in the early count.
While the battle has stretched on for well over 12 months, many are starting to wish it could be settled in 12 rounds. Wrestling body WWE has imagined what it would be like if the candidates just settled their differences in the ring.
See the animation here:
All three candidates sent the WWE a taped message to be played on primary day:
Senator Obama downplayed the likelihood that he could win in Pennsylvania, a victory that would likely knock Senator Clinton out of the race, but pointed out that he had whittled down her lead in most polls from 20 points to single figures.
"I have come to the conclusion that this race will continue until the last primary or caucus vote is cast. And that's not that far away," he said. "Should I end up being the nominee, the work that we've done here in Pennsylvania I think will be extraordinarily helpful in the general election," he said.
Senator Clinton needs a fresh burst of momentum ahead of the next round of contests in Indiana and North Carolina on May 7 (Australian time), which are followed quickly by the last six voting showdowns of the nominating marathon into early June.
There are 158 pledged delegates up for grabs today, the most of any of the remaining contests.
She played up Senator Obama's significant fundraising edge, which has allowed him to triple her advertising buy in the north-eastern state, according to her campaign's estimates.
"Maybe the question ought to be, why can't he close the deal with his extraordinary financial advantage, why can't he win a state like this one if that is the way it turns out?"
Figures released by the US Federal Election Commission have shown Senator Obama's war chest at the start of April at more than $US51 million ($54 million) to Senator Clinton's $US31 million ($33 million).
Senator Clinton is making her case to Democratic "superdelegates", the party officials who will now effectively crown the nominee, since neither candidate is likely to reach the 2025 pledged delegates needed to win outright.
But Senator Clinton is also unlikely to be able to overhaul Senator Obama's lead in delegates even with strong wins in the remaining contests. She also trails him in the popular vote and the number of states won.
Her only remaining hope is to convince the superdelegates that only she can win a general election match-up against Republican John McCain. Part of that strategy involves staying alive as long as possible and hoping Senator Obama self-destructs.
In an indication Senator Obama knows he will not win today, he is due to move straight on to Indiana later in the night, while Senator Clinton has scheduled a victory party in Philadelphia.
- with AFP correspondents in Philadelphia
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