Throughout the Democratic presidential primary, commentators have kept an eye firmly glued to the fluctuations of nationwide preference polling. While national surveys don't directly influence the allotment of pledged delegates in state-by-state contests, they can still be a factor in the race by providing fodder for candidate narratives -- whether "underdog" or "inevitable nominee" -- as well as weighing on the minds of undecided superdelegates.

For most of April, Barack Obama has led Hillary Rodham Clinton in national polling of Democratic voters. Recent data collected after Clinton's victory in the Pennsylvania primary, however, shows a clear tightening of the race from just a week and a half ago, when Obama enjoyed double-digit margins over Clinton.

For the past several days, Gallup's daily tracking poll has shown a slight slip in the percentage of Democrats favoring Obama and a sharp increase in support for Clinton. For the last two iterations of the survey -- the first two conducted entirely after the results from Pennsylvania were in -- the candidates have been tied at 47 percent.

A second poll [PDF] of Democrats, this one from Newsweek, also shows a narrowing race, although it still shows Obama on top, 48 percent to 41 percent. That's a dramatic shift from a week ago, when Newsweek's pollsters measured Obama's lead at nearly 20 points.

The survey finds other encouraging signs for Clinton supporters. Her favorability has dropped, but not as much as Obama's, and he only leads her on this measure by 6 points. (Her unfavorability remains 9 points higher than his, however.) A 45-percent plurality said Clinton was most likely to improve the economy. And 72 percent of her backers said they supported her strongly, compared with 70 percent for Obama supporters, indicating that the enthusiasm gap that existed earlier in the race seems to have evaporated.

No less important for Clinton's camp, which has shown little compunction over aggressively taking on her opponent, are Obama's liabilities. Forty-two percent said Obama's comments about small-town voters made their opinion of him less favorable. And only half of respondents were able to correctly identify him as a practicing Christian or knew that he did not grow up in a wealthy family -- two measures that, besides indicating a basic unfamiliarity with Obama as a person, could mean he will face more accusations that he is too foreign-seeming or elitist to be president.

But despite the tighter poll numbers, Clinton has some familiar hurdles to overcome if she hopes to win the nomination, not least of which is Obama's seemingly impervious lead in pledged delegates. And there are other national polls that show little change in his lead, such as Sunday's daily tracking poll from Rasmussen, which uses an automated polling process. Trend data from that measure shows Obama maintaining an 8-point edge over Clinton -- essentially unchanged from a week ago.