Hershey, Pennsylvania  -  After six weeks of listening to contentious bids for their support, Pennsylvania voters headed to the polls Tuesday with the fate of Hillary Clinton's presidential ambitions in their hands.

Volunteers for Clinton, 60, and Democratic Party rival Barack Obama, 46, were out before polls opened at 7 am to get backers to the schools and civic centers that serve as voting stations.

Clinton is widely expected to win Tuesday's primary, with her appeal to the large number of blue-collar, conservative Democrats who play a key role in the state's
7-million-plus electorate. She also has a home-field advantage with family ties to the north-east of the state.

But the question is what the margin will be, if she holds on for victory. If she fails to win big in the scramble for Pennsylvania's 158 pledged delegates to the August Democratic Party convention, she will likely be under renewed pressure to quit the race in the interests of party unity.

If she surprises with a strong victory, then she and Obama face contests in another seven states and two US territories that don't wrap up until June.

Obama leads slightly in the delegate count but still hasn't reached a majority.

John McCain wrapped up the centre-right Republican presidential nomination last month and will face the Democratic nominee in the November 4 general elections. (dpa)