Pennsylvanians vote in Democratic primary

By John Whitesides, Political Correspondent

WASHINGTON (Reuters) - Hillary Clinton sought a convincing win to keep her flickering White House hopes alive on Tuesday as Pennsylvania Democrats chose between her and Barack Obama in their presidential nominating contest.

Polls close at 8 p.m. (midnight GMT) with results expected shortly thereafter.

Clinton, a New York senator, is favored in Pennsylvania but needs a substantial victory to gain ground on Obama in the Democratic nomination race and convince party leaders she is the best candidate to face Republican John McCain in November's presidential election.

Clinton spent the final day of the increasingly sharp fight touting her strength. She released an ad featuring images of al Qaeda leader Osama bin Laden and warned in an interview with ABC that, as president, she could "totally obliterate" Iran if it attacked Israel.

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.

"This was always an uphill climb but what we feel is that we've made significant progress," Obama said at a diner in Pittsburgh. "A lot of it is going to depend on turnout today and it's really hard to gauge."

Turnout was heavy at many polling places, according to local media. A record number of Pennsylvanians are registered to vote in the state's first contested Democratic primary since 1976 and one poll showed most new registrants plan to vote for Obama.

The Pennsylvania vote opens the final phase of Clinton and Obama's hard-fought duel for the nomination. Nine more contests are scheduled before the campaign concludes on June 3.