While polling data on the rich is imprecise given their small population, polls of the top-earning households favor Hillary Clinton over Donald J. Trump two to one. The July Affluent Barometer survey by Ipsos found that among voters earning more than $100,000 a year — roughly the top 25 percent of households — 45 percent said they planned to vote for Mrs. Clinton, while 28 percent planned to vote for Mr. Trump. The rest were undecided or planned to vote for another candidate.

The spread was even wider among the highest earners. For those earning $250,000 or more — roughly the top 5 percent of households — 53 percent planned to vote for Mrs. Clinton while 25 percent favored Mr. Trump. The survey’s margin of error was plus or minus four points.

It is unclear whether those patterns have changed since July. But if the numbers hold, historians and wealth experts say that next month’s elections may prove to be the largest vote by the wealthy for a Democratic candidate in recent history.

“This would be a big deal, and fundamentally different from what we’ve seen before,” said Andrew Gelman, professor of statistics and political science at Columbia University and co-author of the book “Red State, Blue State, Rich State, Poor State: Why Americans Vote the Way They Do.”