A prominent pollster, one of the few to correctly predict Donald Trump’s victory in 2016, warned that caustic rhetoric like that used by President Biden during a speech at Philadelphia earlier this month will make accurate polling of GOP voters ahead of the midterms “virtually impossible.”

The chief pollster of the Atlanta-based Trafalgar Group, Robert Cahaly, said Mr. Biden’s rhetoric in his Red Sermon — in which he angrily denounced Trump-loving Republicans as a threat to American democracy — is driving GOP voters underground, making accurate polling of their intentions difficult. He predicted that Republican voter turnout will be higher than current models are forecasting and that polls showing growing tailwinds for the Democrats might be overstated.

“The Biden administration has essentially classified ‘MAGA Republicans’ as a threat to democracy, marshaling federal law enforcement to focus on them,” Mr. Cahaly wrote in a weekend Twitter thread. “This move has created a new type of voter that will be even harder to poll or even estimate.”

After months of polls suggesting a Republican rout in November, elections forecasters such as FiveThirtyEight have in recent weeks shifted their prognoses a little more in the Democratic direction. FiveThirtyEight now gives the Democrats an edge in the race for control of the Senate and the Republicans an edge in the House.

Mr. Cahaly said we may be witnessing a repeat of the mistakes made by pollsters ahead of the 2016 and 2020 elections. Pre-election polls in 2016 dramatically underestimated Mr. Trump’s performance in several key battleground states — contributing to the shock of his eventual victory — and polling ahead of the 2020 vote was off by the highest margin in 40 years for the national popular vote, according to the American Association for Public Opinion Research.

Hillary Clinton’s use of the term “deplorables” to refer to Trump supporters, along with the media’s portrayal of them in an unfavorable light, contributed to those polling errors, Mr. Cahaly argued. It was a major contributor, he said, to the “shy Trump voter” phenomenon that caused pollsters to undercount his support and “resulted in a major loss in public confidence for polling following the election.”

A similar phenomenon occurred in 2020, he said, when conservatives who opposed the progressive left’s “woke” agenda hunkered down and avoided expressing their preferences and positions to pollsters and others. It led to so-called hidden voters that many pollsters never reached, undercounting support for Mr. Trump in several key states.

This year may be shaping up a repeat of those performances, especially in light of the dark, red-hued Philadelphia speech in which Mr. Biden said his GOP opponents are dominated by MAGA Republicans and are a threat to the country. “Donald Trump and the MAGA Republicans represent an extremism that threatens the very foundations of our republic,” Mr. Biden said in the speech.

Mr. Cahaly said pollsters like himself had a hard time getting Republicans to be honest in their leanings during the 2016 election cycle because of such negativity. In 2020, many of them stopped responding to pollsters altogether because of the atmosphere. The only Republicans willing to be open with strangers calling them at home — professional pollsters, in other words — were anti-Trump Republicans, which contributed to the polling errors.

“I call this new group ‘submerged voters,’” Mr. Cahaly said. “They aren’t putting stickers on their cars, signs in their yards, posting their opinions, or even answering polls. At this point I think it’s fair to say that Biden’s pursuit of, and attacks on ‘MAGA Republicans,’ has created an army of voters who will be virtually impossible to poll (even for us) and more difficult still to estimate.”

These voters, he added in a subsequent appearance on CNN, “are in their submarines underwater and they are not coming up until election day.