Former President Trump and Vice President Harris are neck and neck heading into their first — and possibly only — debate in Philadelphia on Tuesday, according to a new poll of voters, prompting some pollsters to call it a “warning sign” for Democrats.

Harris holds a 49 percent lead over Trump, 48 percent, among registered voters, a sign that the race has become tighter since August, when Harris held a 3-point lead over Trump, according to an NPR/PBS News/Marist National poll released Tuesday morning.

Among those who definitely plan to vote, the poll found Harris with a 3-point lead over Trump: 51% to 48%.

However, Trump holds a 49% lead over Harris among independent voters, compared to 46% — a 14 percentage point shift from August, when Harris held an 11 percentage point lead in a field that included multiple candidates, such as Robert F. Kennedy Jr., the poll found.

LIVE UPDATES: KAMALA HARRIS, DONALD TRUMP MEET IN PHILADELPHIA FOR TONIGHT’S PRESIDENTIAL DEBATE

New polls show former President Trump and Vice President Harris are in a neck-and-neck race ahead of their presidential debate in Philadelphia on Tuesday. (Emily Elconin/Getty Images, Archive)

Republican pollster Whit Ayers told PBS News that the shift in support for independent candidates is a “warning sign” for Harris’ campaign.

According to some pollsters, the “honeymoon period” for 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, Archive)

“She’s had the honeymoon period,” Ayers said of the initial excitement Harris created among Democrats after President Biden dropped out of the race.

The latest results of the New York Times/Siena poll, released Sunday, also showed the two candidates locked in a neck-and-neck race.

The poll found that Trump had shaken off his enthusiasm for the Democrats after Harris entered the race, with 48% of likely voters backing him, while 47% said they supported the vice president.

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An NPR/PBS News/Marist National poll also found that Trump had 51% of Hispanic voters, compared to 47% for Harris. That’s a 9 percentage point difference from August.

On the issues, the poll found that a majority of Americans see Trump as better equipped to handle the immigration crisis (53% to 46%), the economy (52% to 48%) and the Middle East (51% to 47%). However, Harris is seen 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 a “huge sugar rush” because she was “a new candidate that wasn’t named Trump or Biden.” Madden said recent polls now show that surge of enthusiasm is waning.

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“This race is back to where it was before, which was a very, very close race, among a very divided electorate. 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.

Michael Lee of Fox News Digital contributed to this report.