PRINCETON, NJ -- Democratic presidential nomination rivals Barack Obama and Hillary Clinton will debate tonight in Philadelphia before next Tuesday's crucial Pennsylvania primary. The latest Gallup Poll Daily tracking update shows Obama with a 50% to 42% lead over Clinton in national Democratic voters' nomination preferences.

This marks the 10th consecutive day that Obama has held a statistically significant lead over Clinton, easily his best stretch of the campaign. (To view the complete trend since Jan. 3, 2008, click here.) That included a campaign best 11 percentage point lead in Tuesday's release. Obama's current margin is down slightly, but still in line with the average 9-point lead in this recent stretch.

Both Democrats hold an advantage over John McCain in registered voters' presidential election choices. In an Obama-McCain match up, 46% of registered voters say they would vote for Obama if the election were held today and 43% would choose McCain. The Clinton-McCain trial heat has Clinton at 46% and McCain at 44%.

Obama has now held at least a slim advantage over McCain for the last nine days, and Clinton has done so for six days. The fact that the Democratic candidates' margins over McCain are their best to date underscores the closeness of the election. McCain has led the Democratic candidates by as many as four percentage points. -- Jeff Jones

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 13-15, 2008. For results based on this sample of 1,282 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 11-15, 2008. For results based on this sample of 4,435 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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