(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.

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. [MORE]