Vice President Kamala Harris’ growing lead in the polls may be exaggerated by oversampling Democratic voters and some critics say it’s an intentional bid to bolster her momentum.

Democrats are celebrating after Ms. Harris closed the gap in polls with former President Donald Trump and even surpassed him nationally and in some battleground states.

Just a few weeks ago, Mr. Trump held a consistent lead in nearly every critical poll in a matchup with President Biden. But Ms. Harris has more than caught up with Mr. Trump since replacing Mr. Biden on the ticket on July 21.

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She is beating Mr. Trump nationally by nearly 2 points and leading slightly or tying Mr. Trump in all seven battleground states.

GOP analysts say polling methodology gives Ms. Harris a phantom advantage because many polls sample far fewer Republicans than the number of GOP voters who participated in the 2020 presidential election according to exit polls.

In other words, say critics, the polls oversample Democrats, perhaps purposely, to generate enthusiasm and to boost fundraising for Ms. Harris. Last week, the Harris campaign announced it had raised $540 million in July, more than four times the money raised by Mr. Trump in the same period. 

Mr. Trump, who is now confronted with a dead-heat race with Ms. Harris after leading Mr. Biden for weeks, mocked the oversampling of Democrats in the latest polls.

“It’s fake news,” Mr. Trump said during a rally in Michigan. “They can make those polls sing.” 

Ms. Harris’s advancement in the polls has undoubtedly fueled the excitement and donations. 

Ms. Harris opened up a five-point lead over Mr. Trump in a new Suffolk University/USA Today poll released on Thursday. The 48%-43% results in favor of Ms. Harris represent an 8-point turnaround from June, when Mr. Trump was 4 points ahead of Mr. Biden.

Like several other major polls showing a Harris lead, the survey included more answers from those identifying as Democrats, who made up 37.10% of respondents, than Republicans, who comprised 33.8%, according to the data. Among those polled, 48% said they voted for Mr. Biden in 2020, while 41.6% said they picked Mr. Trump. 

Democrats outnumbered GOP respondents in another poll, released August 27 by Yahoo News and YouGov, which found Ms. Harris ahead of Mr. Trump, 47% to 46%. 

Among those who participated in the poll, 33% identified as Democrats and 29% as Republicans. The percentage of GOP participants was significantly below the 2020 exit polling in which 36% of voters identified as Republican. 

In the same 2020 exit poll, 37% of voters identified as Democrats, a mere 1-point advantage over the GOP. 

The Yahoo/YouGov poll showed Mr. Trump winning independents by a margin of 44% to 35% over Ms. Harris. The former president enjoyed a 17-point advantage with seniors in the poll and picked up 13% of Black voters and 39% of Hispanic voters. 

Mr. Trump’s poll numbers “are all significantly better than 2020,” Jim McLaughlin, who polls for the former president, said. “How can they have Donald Trump losing? It’s simple. They undersample Republicans.” 

The skewed sample, Mr. McLaughlin said, aims to “tamp down support and donations for Trump.”

Skeptics of Harris’s poll bounce question how she can climb in the polls without significantly raising low approval ratings that have dogged her as vice president, which are also the lowest job-approval ratings for a vice president in modern U.S. history. Those approval ratings have only improved slightly since Ms. Harris began running for president on July 21.

The polling analysis site FiveThirtyEight calculates a 42.3% approval rating for Ms. Harris, up from 37.1% on July 7.  

The latest polls are drawing comparisons to voter surveys taken ahead of the 2016 and 2020 elections, which showed far greater support for the Democrats than the final results. The 2016 polls predicted a win for Democrat Hillary Clinton and the 2020 polls forecast a much bigger win for Mr. Biden, whose victory was extremely narrow. 

Suffolk Poll Director David Paleologos defended the proportion of Democrats and Republicans sampled in the latest Suffolk/USA Today poll. The Democrat voter advantage, he said, is nearly identical to the advantage recorded in the 2016 presidential election exit polls, which he said more likely mirrors the 2024 election due to the pandemic’s impact on the 2020 election. 

As for the poll’s inclusion of more Biden voters, Mr. Paleologos said it is not accurate to directly compare those numbers to 2020 because the sample includes people who didn’t vote, picked a third-party candidate or refused to say how they voted. 

Those who conducted the Yahoo/YouGov poll said party identification was based on “the respondent’s most recent answer given prior to Nov. 1, 2022 … and weighted to the estimated distribution at that time, which was 33% Democratic, 27% Republican.”

The poll selected respondents from YouGov’s “opt-in panel” to be representative of all U.S. adults, and the poll’s margin of error is approximately 2.7%, which is considered very reliable.

Other prominent polls surveyed fewer 2020 Trump voters than the numbers recorded in 2020 presidential election exit polls.

The August New York Times/Siena Poll sampled fewer Trump voters by several percentage points when compared to 2020 results in the key battleground states of Pennsylvania, Wisconsin and Michigan.

In Wisconsin, for example, 44% of voters surveyed said they voted for Mr. Trump in 2020, nearly 5% fewer voters than the 49% the former president ended up winning in the Badger State nearly four years ago. Mr. Biden won less than 50% of Wisconsin voters in 2020 but in the poll, 52% of respondents said they voted for Mr. Biden.

The poll found Ms. Harris leading Mr. Trump by four points in all three states.

“They have dramatically understated President Trump’s support both among all registered voters and in their likely-voter model. In each state, the gap between the survey’s recalled 2020 vote and the reported 2020 election results is more than the margin between Kamala Harris and President Trump,” Mr. Trump’s chief pollster, Tony Fabrizio, said in an internal memo. “Once again, we see a series of public surveys released with the clear intent and purpose of depressing support for President Trump.”

Siena Poll Director Don Levy said asking respondents who they voted for in the prior election has been “historically unreliable,” and is not an indication that Democratic voters were oversampled for the survey. He acknowledged that pollsters are grappling with possible “response bias,” or over-participation of Democrats who are more willing to take part in the polls and under-participation from Trump supporters who hang up. 

Mr. Levy said that despite sampling issues, it’s clear Ms. Harris has closed the gap with Mr. Trump since she entered the race. 

“You can criticize the sampling methodology but if you look at the trend lines, the current movement in the race is undeniable,” he said. 

Mr. Paleologos played down the oversampling influence on recent polls, pointing out that close to 20 states do not even require voters to register by party.

“There is no fixed amount of registered Democrats and Republicans in the United States,” Suffolk University Polling Director David Paleologos said. “Party Identification is a function of many factors, including excitement for candidates, other races or ballot questions in the state. In short, it is a fluid, moving target.”

For example, Mr. Paleologos pointed out that Democrats polled 3 points ahead of Republicans in the exit polls taken during the 2016 presidential election, which Mr. Trump won. 

YouGov and New York Times/Siena polls are ranked among the nation’s most accurate. 

Other analysts, however, have raised questions about recent polls showing Ms. Harris in the lead. 

Ryan James Girdusky, a political and polling analyst who writes on substack, pointed out that two new polls by Fairleigh Dickinson University and NPR/Marist College, show Ms. Harris leading Mr. Trump among senior voters by 16 points and 11 points, respectively.

Mr. Girdusky said the advantage for Ms. Harris among seniors is an example of response bias, in this case, concentrated participation by older white liberal voters.  

The same bias contributed to polling that inaccurately undercounted senior support for Mr. Trump in both 2016 and 2020. 

“With less than 70 days to go, Kamala Harris’s marginal lead in polls is built off the back of a group she will lose, and probably somewhere in the margin Biden and Hillary lost: four to nine points,” Mr. Girdusky wrote in The American Conservative.

The FDU poll showed Mr. Trump trailing Ms. Harris nationally by 7 points. Democrats and voters leaning Democrat made up 42.3% of respondents, while 38% identified as GOP voters and leaners. Independents made up 13.1%. 

But the poll is not necessarily overselling support for Harris, FDU Poll Director Dan Cassino said. 

“The question is only meaningful if we assume that partisanship is set, that some people are Republicans and some people are Democrats, and they never change,” Mr. Cassino said. “In fact, people move around — especially between the independent and leaner categories — all the time. All this tells me is that, when we were in the field, more independents were leaning toward the Democrats. Next week, those same people could be independents or leaning towards the Republicans.”

As for the poll’s big senior support for Harris, Mr. Cassino said subgroups, including seniors, have a higher degree of sampling error, and that pollsters weight their calculations to try to eliminate those errors. 

“We don’t have any reason to believe that older Trump voters, in particular, would be less likely to take a survey, or that older Harris voters would be more likely, but we’d have no way of knowing if that were happening. Weighing can solve some of these issues, but not all of them,” Mr. Cassino said. “In general, is it possible that Democrats in that group were more likely to pick up the phone and do a survey? Sure. Much of our field time was during the Democratic National Convention, and so while we didn’t pick up on higher response rates among Democrats while we were in the field — we’ve never seen a problem with shy Trump voters — it’s certainly possible.”