Some 21% of respondents in the three-day poll, which closed on Sunday, said "political extremism or threats to democracy" was the biggest problem facing the U.S., a share that was marginally higher than those who picked the economy - 19% - and immigration - 18%.

Biden's Democrats considered extremism by far the No. 1 issue while Trump's Republicans overwhelmingly chose immigration.

Extremism was independents' top concern, cited by almost a third of independent respondents, followed by immigration, cited by about one in five. The economy ranked third.

During and since his presidency, Trump has kept up a steady drumbeat of criticism of U.S. institutions, claiming the four criminal prosecutions he faces are politically motivated and holding to his false claims that his 2020 election defeat was the result of widespread fraud.

That rhetoric was central to his message to supporters ahead of their Jan. 6, 2021, assault on the U.S. Capitol.

Overall, 34% of respondents said Biden had a better approach for handling extremism, compared to 31% who said Trump, the frontrunner for the Republican presidential nomination.

The poll helps show the extent to which Biden's re-election bid could rely on voters being motivated by their opposition to Trump rather than enthusiasm over Biden's candidacy.

BIDEN APPROVAL DIPS

Biden's approval rating in the poll, 37%, was close to the lowest level of his presidency and down a percentage point from a month earlier. Nine-out-of-ten Democrats approved of his performance and the same share of Republicans disapproved, while independents were slightly skewed toward disapproval.

But 44% of Democrats said extremism was their top issue, compared to 10% who said the economy, their second most-picked concern. Prior Reuters/Ipsos polls did not include political extremism as an option for respondents to select as the country's biggest problem.

Biden's re-election campaign has focused its messaging on the dangers to democracy posed by Trump, whose many legal problems include criminal charges tied to his efforts to overturn his loss to Biden in the 2020 presidential election. Other Reuters/Ipsos polls have shown Biden's supporters are more motivated by their opposition to Trump than by their support for the president.

Trump has pleaded not guilty to all the charges he faces, which he claims are part a conspiracy by Democrats to derail his return to the White House.

Trump has regularly launched verbal attacks against the prosecutors and judges handling his civil and criminal cases, and a Reuters review earlier this month found that serious threats to U.S. federal judges have more than doubled over the past three years.

While 38% of Republicans in the poll cited immigration as the top issue for the country, a significant proportion - 13% - picked extremism, a sign that Trump's own claims about the danger to the nation posed by "far left" Democrats also resonate with his base.

The economy, which has suffered under high inflation for most of Biden's presidency, was the second biggest issue among Republicans, with 22% saying it weighed the most.

The economy has long been a sore spot for Biden. Thirty-nine percent of poll respondents said Trump had a better approach to the economy, compared to 33% who said Biden did.

Trump led Biden 36% to 30% when it came to having a better approach to foreign conflicts, though few Democrats or Republicans considered those issues to be top national priorities.

The Reuters/Ipsos poll gathered responses online from 1,020 adults, using a nationally representative sample, and had a margin of error of about 3 percentage points.

(Reporting by Jason Lange; Editing by Scott Malone and Daniel Wallis)

By Jason Lange