As polls closed across the eastern state of Pennsylvania on Tuesday, Hillary Clinton was looking for a win to keep alive her presidential bid against Democratic rival Barack Obama. ( dpa )
After six weeks of contentious campaigning for the state's large delegate prize, Senator Clinton, 60, was hoping for a strong enough win against Senator Obama, 46, to withstand demands that she step aside and to gain momentum for the nine remaining contests.
US broadcasters said the race was too close to call after polls closed at 8pm (0000 GMT).
Pennsylvania's 158 delegates are up for grabs, and Clinton's double-digit edge from some weeks ago had been shaved to as little as 5 percentage points by Obama, who has outspent her on advertising more than two-to-one.
Pennsylvania voters turned out in droves with the economy and health care on their minds and the fate of Clinton's presidential ambitions in their hands.
Nationally, Obama has a small lead of 150 delegates in the race to the Democratic nominating convention in August, where 2,024 delegates are needed to reach majority.
Even if Clinton falls short of a significant win in Pennsylvania, she has indicated she won't back down, and her stance was welcomed by her supporters at a polling station in Hershey.
"Sometimes we've got to go through pain to gain, and if it's for the right person to be the nominee, it's absolutely necessary," said Suzanne Brinser, 40, after voting for Clinton.