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President Donald Trump's support has eroded among key groups of voters who backed him in 2016 - a major reason why he continues to trail former Vice President Joe Biden and a prime motivator for the president's re-election strategy of emphasizing violent disorder in the nation's cities.

Trump's decline among parts of his 2016 base is a chief finding so far from the USC Dornsife Daybreak Poll, which tracked voter preferences daily four years ago and is doing so again this year. Overall, Trump has lost support from about 9 per cent of voters who backed him in 2016, the poll finds.

The poll shows no major shift in the race during the last two weeks, belying much speculation that the back-to-back national political conventions and violence in Portland, Oregon, and Kenosha, Wisconsin, might have changed what has been an unusually stable contest.

During the Democratic convention, Biden gained 2 percentage points and Trump lost 2 points; Trump then regained some of that ground during his convention, a week-by-week comparison of the poll's tracking shows.

The net result is a Biden lead of 11 points, 52 per cent to 41 per cent, in the poll's latest results as of Monday, after the Republican convention. A rolling average of results over the last week has been virtually the same, 53 per cent to 41 per cent.

"Independents who lean toward the Republican Party seem to have been temporarily swayed by Biden's message" during the Democratic convention, said Jill Darling, the survey director for the USC Dornsife poll. That widened Biden's lead for a bit. But "Trump's dark view of the Democrats' agenda seems to have swept them back into the fold, so the overall result is pretty much a wash."

Biden's lead is almost double the 6-point lead that the poll showed for Hillary Clinton at this point four years ago. Clinton was then on a downward track, declining from the large boost she had received from her convention.

That comparison to four years ago is based on an adjusted version of the 2016 USC Dornsife survey. The poll significantly overstated Trump's support. It showed him leading by October when, in fact, he lost the nationwide vote to Clinton by more than 2 percentage points.

An analysis after the election showed that the poll, which tracks the views of a panel of more than 8,000 eligible voters, had over-represented rural Americans, a group who ended up leaning heavily toward Trump. The 6-point lead for Clinton represents her margin at this point four years ago once the poll was re-weighted to fix that mistake.

Another big difference from four years ago: Third-party candidates get only 2 per cent support now, much less than in 2016, when smaller parties took 5 per cent of the vote.

While a record number of Americans - perhaps as many as 150 million - are expected to vote in this fall's elections, the vast majority already have made up their minds. Over the next two months, the two sides will pour huge amounts of time and money into maximizing turnout among their supporters. At the same time, they'll battle over the small slice of the electorate that remains up for grabs.

The poll's findings help pinpoint who those wavering voters are - mostly independents, who are closely divided between Biden and Trump, and less partisan Republicans who may have supported Trump in the past, but have soured on him since.
Democrats and independents who lean Democratic have largely consolidated behind Biden, the poll finds.

Democrats and their allies have tried to win over swing voters by stressing Trump's failure to control Covid-19 and the personality traits - belligerence, lack of empathy and flouting of traditional norms for presidential behavior - that many of those voters dislike.

Trump has countered with dire warnings about violence in American cities and thinly veiled appeals to racial prejudice, especially tied to white fears of Black families moving into predominantly white suburbs. He has depicted Biden as a "puppet" of "radical Democrats."

Analysis of which voters Trump has lost since 2016 provides insight into the problem his reelection campaign faces. Because the USC Dornsife panel includes many people who responded to the poll in 2016, its findings provide direct evidence of which voters have changed their minds.

Although Trump has lost about 9 per cent of his 2016 voters, about 4 per cent of those who voted for Clinton now say they will vote for Trump.

That gives Trump a net loss of 5 percentage points, which may not seem a big number, but sets up a high hurdle for an incumbent who barely eked out victory four years ago, winning by less than 1 point in each of the three states that put him over the top.

"You can't lose 9 per cent or 10 per cent of the people who voted for you last time" and still win, said Bob Shrum, the director of USC Center for the Political Future, which co-sponsors the poll, and a veteran Democratic strategist. "If that holds, it would be catastrophic for him."