WASHINGTON (Reuters) - Barack Obama is gaining ground on Hillary Clinton in Pennsylvania, moving within range of an April 22 upset that could end the hard-fought Democratic presidential race months earlier than expected.

Democratic presidential candidate Barack Obama (D-IL) jokes with photographers before hearing the testimony from Commander in Iraq General David Petraeus and US Ambassador to Iraq Ryan Crocker to the Senate Foreign Relations Committee on Capitol Hill in Washington April 8, 2008. REUTERS/Joshua Roberts

Less than two weeks before their next showdown, opinion polls show Obama cutting Clinton’s once big lead in Pennsylvania to single digits and making gains among some voting blocs that have been her most reliable backers.

An Obama win would be a shocking twist in a Democratic race filled with them, effectively scuttling Clinton’s hopes of overtaking him in the fight for the right to face Republican John McCain in November’s presidential election.

“If Obama wins Pennsylvania, the race is over,” said Cal Jillson, a political analyst at Southern Methodist University in Texas.

“That would be a signal to the Democratic Party to move decisively to end this nominating race now and to start looking to the general election,” he said.

A loss in Pennsylvania would destroy Clinton’s central argument in the race against Obama -- that she is the strongest Democratic candidate in the big states the party needs to win in the election battle with McCain.

Pennsylvania had been considered a lock for Clinton because of its high concentration of the older, white, Catholic and blue-collar voters that form the backbone of the New York senator’s coalition.

But a heavy barrage of television advertising by Obama, who has been tripling Clinton’s spending in Pennsylvania, a successful six-day bus tour of the state and a few bad weeks for Clinton have helped tighten the race, analysts said.

A new Quinnipiac University poll showed Clinton’s lead at 6 points, down from 9 points last week and 12 points in mid-March. An average of all polls in the state this week shows Clinton with about a 7-point advantage.

‘OBAMA CLOSES IN’

“Obama is definitely closing in. You look at all the demographic groups that have been her base and she is slipping everywhere,” said Clay Richards, assistant director of the Quinnipiac University poll.

But Obama has closed the gap on Clinton ahead of contests in big states like Ohio, New Jersey, California and elsewhere -- only to lose. It was common for polls to tighten after heavy advertising campaigns like Obama’s and as the voting draws closer, Clinton aides said.

The Quinnipiac poll showed Obama, an Illinois senator, gaining ground among whites, women and those voters who rank the economy as their top issue, he said. Obama also was picking up support in the populous and crucial Philadelphia suburbs.

Terry Madonna, a political analyst at Franklin and Marshall College in Pennsylvania, said Obama’s advertising had been effective and his week-long swing through the state showed him talking with voters in a more personal and effective style.

“He was talking specifics in ways that he didn’t before, about health care and the economy -- it wasn’t just ‘turn the page,’” Madonna said.

“He started really talking to people about the problems they face. In other words, he was doing what Hillary usually does so well,” he said.

Meanwhile, Clinton suffered through questions about her disproven claims to have faced sniper fire in Bosnia in 1996 -- she said she “misspoke” -- and whether she should even remain in the race amid dwindling hopes of catching Obama.

Obama has a nearly unassailable lead on Clinton in the pledged delegates who will help pick the nominee, but neither is likely to reach the 2,024 needed to clinch the nomination once the nominating contests end in early June.

That leaves the decision to nearly 800 superdelegates -- office holders and party insiders free to back any candidate. Clinton has courted them by saying her wins in Ohio, California, New Jersey and elsewhere proved her strength in big states critical to beating McCain.

She also hopes the final contests will help her close the gap on Obama in pledged delegates and catch him in popular votes cast in the state-by-state contests. All that is gone with a loss in Pennsylvania -- or possibly even a narrow win, Madonna said.

“If he gets within 5 percentage points of her, he can declare victory anyway. That may not get her out of the race but it deflates her arguments,” Madonna said.

Clinton spokesman Howard Wolfson rejected the notion she has to win big in the state to make meaningful headway.

“A win is a win. There is no reason why Senator Obama can’t win there. If they fail to win there, they will have failed in Pennsylvania,” he said.

(Editing by David Wiessler)