Hawkfish, a Democratic data firm funded by billionaire former New York Mayor Michael Bloomberg, expects far more Democrats to vote by mail than Republicans. That expectation is based on a July 1-Aug. 16 survey of 17,263 registered voters nationwide, which measured how people intend to vote and who they intend to vote for.

Because it could take days or weeks to tally the mail-in ballots, that could create the effect of making Trump look like he is in the lead on election night.

“We are sounding an alarm and saying that this is a very real possibility, that the data is going to show on election night an incredible victory for Donald Trump,” he said.

“When every legitimate vote is tallied and we get to that final day, which will be someday after Election Day, it will in fact show that what happened on election night was exactly that, a mirage,” Mendelsohn said. “It looked like Donald Trump was in the lead and he fundamentally was not when every ballot gets counted.”