Former President Trump and Vice President Harris are neck and neck ahead of their first – and potentially only – debate in Philadelphia on Wednesday, according to a new survey of voters that has some pollsters seeing a "warning sign" for Democrats.

Harris leads Trump 49% to 48% among registered voters, showing a race that has tightened since August when Harris held a 3-point lead against Trump, according to an NPR/PBS News/Marist National Poll released Wednesday morning.

Among those who say they definitely plan to vote, the survey found Harris edging Trump by 3 points, 51% to 48%.

Trump, however, leads Harris among Independent voters 49% to 46% – a 14-point shift from August when Harris commanded an 11-point lead in a field that had multiple candidates, such as Robert F. Kennedy Jr., the poll found.

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New polling shows former President Trump and Vice President Harris locked in a dead heat ahead of their presidential debate in Philadelphia on Wednesday. (Emily Elconin/Getty Images, File)

Republican pollster Whit Ayers told PBS News that the swing in independent support is a "warning sign" for Harris’ campaign.

Some pollsters say that the "honeymoon period" for the Harris campaign is over as support for the candidates appears neck and neck after an initial surge of enthusiasm for the Democrat. (Sarah Rice for The Washington Post via Getty Images, File)

"She had the honeymoon period," Ayers said of the initial excitement that Harris generated among Democrats after President Biden exited the race.

The latest results of the New York Times/Siena poll released on Sunday also found the two candidates locked in a dead heat.

In that poll, Trump seemingly shrugged off a burst of enthusiasm for Democrats after Harris entered the race, garnering the support of 48% of likely voters, compared to 47% who indicated support for the vice president. 

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The NPR/PBS News/Marist National Poll also showed Trump up with Latinos voters 51% versus 47% for Harris, a 9-point swing since August.

On the issues, the survey found that a majority of Americans consider Trump better equipped to handle the immigration crisis (53% to 46%), the economy (52% to 48%) and the Middle East (51% to 47%). Harris, however, is perceived as the candidate to better handle the abortion issue, 56% to Trump’s 42%.

Republican strategist Kevin Madden told PBS News that Harris’ initial surge of support and enthusiasm was an "enormous sugar high" because she was "a new candidate not named Trump or Biden." Now, Madden says recent polling shows that burst of excitement subsiding.

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"This race has snapped back to where it always was, which is a very, very close contest, amongst a very bitterly divided electorate, and that this election is going to be very, very close and it’s going to come down to a few hundred thousand voters in a handful of states," Madden told the outlet.

Fox News Digital’s Michael Lee contributed to this report.