If voters were indeed afraid of voicing their support for the president, Mr. Winston said, other numbers in the poll would reflect that, like seeing an uptick in the percentage of undecided voters rather than a rise in support for Mr. Biden. “It would not be people saying they are voting for Biden,” he said, “but that they’re undecided.”

While the effects of a hidden Trump vote are certainly overstated by the president’s allies, that does not mean that no evidence exists that polls are missing some of his voters. A small percentage of his support is probably being undercounted, and has been in the past, public opinion experts said. And in states like North Carolina, where the margin of victory could be narrow, the undercount could make a difference between a poll being right or wrong…

Mr. Newhouse has picked up further evidence of such reluctance recently. In polls he conducted late last month in North Carolina and Iowa, he found that one-quarter to one-third of voters answered “yes” when asked if they knew someone who is voting for Mr. Trump but would not say so to anyone but their closest friends.

“This totally confirms the notion of ‘shy Trump voters,’” Mr. Newhouse said. But, he added, if polls are undercounting some Trump voters — a group that tends to be uniquely expressive and adamant about their support for the president — no one can say by how much.