Vice President Kamala Harris’ momentum since taking over the top of the Democratic presidential ticket reached a high in a new poll Saturday that showed her leading former President Donald Trump in three battleground states. 

The latest poll from The New York Times and Siena College showed that Ms. Harris leaped to a four-point lead in Pennsylvania, Wisconsin and Michigan. Of the voters surveyed in those states, 50% said they would back the vice president, while 46% said they would vote for Mr. Trump. 

The survey showed a strong turnaround for Democrats’ chances of keeping the White House in November after President Biden’s incoherent debate showing in June appeared to spell doom for his reelection. 

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Ms. Harris has been riding a wave of momentum since Mr. Biden dropped out of the race, but Republicans argue that her honeymoon phase will end by next month. Still, in the three weeks since taking over the top of the ticket, she has outperformed Mr. Biden in every match-up with Mr. Trump. 

In two New York Times and Siena polls from earlier this year, Mr. Biden trailed Mr. Trump by three points in Pennsylvania, which is where the president was born and the site of his then campaign headquarters. 

Mr. Biden also trailed Mr. Trump in Wisconsin by one point while maintaining a tenuous grip on a one-point lead in Michigan. 

When third-party candidates, including Robert F. Kennedy Jr., were added to the mix, voters in the battleground states still supported Ms. Harris over Mr. Trump 48%-43%. 

Mr. Kennedy siphoned off about 5% of support from either candidate, Cornel West and Jill Stein scored half a percent and 1% of support, respectively. 

The latest poll surveyed 619 voters in Michigan and 661 in Wisconsin from Aug. 5-8, while 693 voters in Pennsylvania were surveyed from Aug. 6-9.

Democrats, by 87%, were satisfied with their party’s nominee versus 79% Republicans.