Kathy Boockvar, a former top election official for Pennsylvania, said fears of voter intimidation and violence run counter to American tradition.

"Our country is based on democracy. We should be excited about Election Day," said Boockvar, a member of the bipartisan Committee for Safe and Secure Election.

Distrust between America's two political camps has grown over the last half century, with bipartisan legislation becoming rarer and a growing share of parents saying they would be displeased if their child married someone from the other political party.

Among the registered voters polled by Reuters/Ipsos, 43% were concerned about threats of violence or voter intimidation while voting in person. The fear was more pronounced among Democratic voters, 51% of whom said they worried about violence, although a still-significant share of Republicans - 38% - harbored the same concerns.

About a fifth of voters - including one in 10 Democrats and one in four Republicans - said they were not confident their ballots would be accurately counted.

Fired up by his false fraud claims, thousands of Trump supporters stormed the Capitol on Jan. 6, 2021.

While voter rights advocates accuse far-right groups who believe those claims of sending poll watchers to intimidate minority voters aligned with the Democratic Party, U.S. conservative media highlight left-wing violence, frequently tying Democrats to riots sparked by the 2020 murder of George Floyd, a Black man, by a white police officer in Minneapolis.

Some two-thirds of registered voters - 67% - said they were concerned extremists will commit acts of violence after the election, including about three in four registered Democrats and three in five registered Republicans.

More than 10 million people have already cast ballots in the contests that will shape the rest of Democratic President Joe Biden's term.

Republican control of either chamber of Congress would effectively torpedo Biden's agenda.

About two-thirds of Republicans and one-third of Democrats think voter fraud is a widespread problem, the Reuters/Ipsos poll found. Two-thirds of Republicans think the 2020 presidential election was stolen from Trump.

Trump's claims of fraud were dismissed by dozens of U.S. courts, state reviews and multiple members of his administration. Nonetheless, they have found widespread acceptance, helping fuel a cottage industry of poll-watching tools.

One software application heavily promoted by far-right media organizations lets users view a map of reported polling station problems and abnormalities in vote counts. Conservative activists have set up a hotline to collect similar reports.

The Reuters/Ipsos online poll gathered responses from 4,413 U.S. adults nationwide and had a credibility interval, a measure of precision, of between 2 and 5 percentage points.