Hillary Clinton was engaged in a do or die battle to keep her White House hopes alive as Democrats in Pennsylvania went to the polls on Tuesday to choose their nominee in an increasingly bitter contest.
The former first lady is favoured to win, but needs a convincing victory over frontrunner Barack Obama to cut into his lead in the overall delegate count and the popular vote. Obama leads her by 1,648 to 1,504 delegates, who pick the party nominee.
Polls close at 8 pm (5.30 am IST on Wednesday) with results expected shortly thereafter.
Unless she wins a substantial number of the 158 delegates at stake in Pennsylvania, Clinton would be hard put to win the support of superdelegates for her claim that she is the best candidate to face Republican John McCain in November's presidential election.
Neither candidate is expected to pick up the 2,025 delegates needed to win the Democratic nomination at the end of the primary season June 3. That would leave it to the Democratic superdelegates to decide the party nominee at a convention in late August.
Superdelegates are Democratic Party delegates or elected officials who can vote at the national convention. They choose their vote and are not required to commit to a particular candidate before the convention.
Clinton's one-time 20-point lead in the state has slipped to single digits in many polls amid an onslaught of advertisements by Obama, who has heavily outspent her in the first nominating contest in six weeks. Both camps tried to play down expectations ahead of the vote.
A CNN "poll of polls" of likely Pennsylvania Democratic primary voters calculated on Monday shows Clinton leading Obama 50 percent to 43 per cent with 7 per cent unsure. The poll of polls is an average of three polls conducted by Zogby, Suffolk University and Quinnipiac University conducted on April 18-20.
In the six weeks since the Mississippi primary, the Democratic race has taken on a particularly negative tone, with both candidates launching waves of robocalls, tough mailers and matching attack ads.
The two candidates also spent millions on television advertising. Since the beginning of the year, the Obama camp has spent over $8.5 million in television spots in the state, while the Clinton camp spent $3.6 million.
Calls for Clinton to drop out of the race have increased from within the party as the two candidates have stepped up attacks on each other. The increasingly pointed attacks have stoked fears among some Democratic leaders that the bruising primary battle could hurt Democratic chances in the general election.
"If Clinton wins by more than 10 points, which was her margin in neighboúring Ohio and New Jersey, her campaign will have new momentum and she will soldier on," said Bill Schneider, CNN senior political analyst. "If Clinton wins by single digits, we're in a political twilight zone. Nothing changes."
Clinton may be helped by the fact that Pennsylvania conducts "closed" primaries, meaning that only registered Democrats will be allowed to vote. Obama has benefited from the support of independent voters in previous primaries.
Clinton may also benefit from the fact that Pennsylvania has the second oldest population in the US, behind only Florida.
A Quinnipiac poll conducted April 18-20 has Clinton leading Obama 54 per cent to 40 per cent among voters 45 and over. Obama leads Clinton 57 per cent to 41 per cent among voters under the age of 45. The poll's margin of error was plus-or-minus 3.1 percentage points.
Obama is expected to do well in Philadelphia, which has a large African-American population, and the suburban counties surrounding Philadelphia, home to many upscale voters that in past primaries have tended to back the Illinois Democrat.
Clinton, on the other hand, is expected to do well in the more blue-collar city of Pittsburgh, located in western Pennsylvania, as well as the largely rural area in the middle of the state.
Pennsylvania will also hold a Republican primary, but as John McCain has already won the 1,191 delegates needed to get the party presidential nomination, it would be merely an academic exercise.