New private polling shared first with POLITICO showed that Republicans have become overwhelmingly concerned about mail balloting, which Trump has claimed without evidence, will lead to widespread voter fraud. A potentially decisive slice of Trump’s battleground-state base — 15 percent of Trump voters in Florida, 12 percent in Pennsylvania and 10 percent in Michigan — said that getting a ballot in the mail would make them less likely to vote in November…

“He’s sowing the seeds of his own downfall with his rhetoric around vote by mail,” said Katie Merrill, a Democratic strategist whose consulting firm, Baughman Merrill, commissioned the polling by FM3 Research, a respected outfit that polls for a range of clients including Democratic campaigns and municipal governments. “It really was like President Trump looked at the crosstabs of our poll when he tweeted. He tweeted exactly to his base what they are thinking.”

Large majorities of swing-state voters expressed concerns about their health, with 70 percent viewing the pandemic as an “extremely” or “very serious” problem. Trump’s supporters skew older than Biden’s, including people who believe their health is at risk. Because of this, pollster Dave Metz said it’s more likely the president’s supporters will shy away from going to the polls in person, particularly if state and local health officials issue warnings against doing so.