PRINCETON, NJ -- As Democratic voters in Pennsylvania cast their presidential ballots today, the latest nationwide Gallup Poll Daily tracking results show Barack Obama with a 10 percentage point lead over Hillary Clinton.

Obama has held a statistically significant lead the last two days after Clinton enjoyed a temporary increase in support following last Wednesday's Philadelphia debate, which brought the race back into a statistical dead heat. Prior to that, Obama led Clinton for 11 consecutive days, with margins ranging from seven to 11 points. (To view the complete trend since Jan. 3, 2008, click here.)

Clinton is expected to prevail in Pennsylvania, according to the pre-election polls, but the pundits are focusing speculation on the size of the margin. A big win could give her campaign a boost and make a comeback seem more possible. A close win would probably not alter the dynamics of the race, and make her campaign's arguments that she can overcome Obama's delegate lead less persuasive. An Obama win could lead to heavy pressure from Democratic Party leaders for Clinton to drop out of the race. Gallup Poll Daily tracking data will begin to show any national impact of the Pennsylvania results on Thursday, since tonight's polling will involve a mixture of interviewing conducted before and after the winner of that contest is known.

Neither Democrat can claim stronger positioning against John McCain at this point. Among registered voters nationwide, McCain and Obama are even at 45%, while McCain outpolls Clinton by a single point, 46% to 45%. -- Jeff Jones

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Survey Methods

For the Gallup Poll Daily tracking survey, Gallup is interviewing no fewer than 1,000 U.S. adults nationwide each day during 2008.

The Democratic nomination results are based on combined data from April 19-21, 2008. For results based on this sample of 1,259 Democratic and Democratic-leaning voters, the maximum margin of sampling error is ±3 percentage points.

The general election results are based on combined data from April 17-21, 2008. For results based on this sample of 4,367 registered voters, the maximum margin of sampling error is ±2 percentage points.

In addition to sampling error, question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of public opinion polls.

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