One year from the 2020 election, Donald Trump trails some potential Democratic rivals in head-to-head matchups, with his national support level fixed at about 40 per cent, according to a Washington Post-ABC News poll.

The new poll highlights the degree to which most of the country already has made a judgment about the president's performance and its voting preferences next year. Among the 39 per cent of registered voters who approve of Mr Trump's job performance, Mr Trump is winning at least 95 per cent support against each of five possible Democratic opponents. But among the 58 per cent of voters who disapprove of Mr Trump, he receives no more than 7 per cent of support.

Former vice president Joe Biden, senator Bernie Sanders and senator Elizabeth Warren run strongest against the president nationally, with Mr Biden leading by 17 points (56 per cent to 39 percent), Ms Warren by 15 points (55 per cent to 40 per cent) and Mr Sanders by 14 points (55 per cent to 41 per cent).

Mayor of South Bend, Indiana Pete Buttigieg and senator Kamala Harris, the other two Democrats tested against Mr Trump, also lead the president among registered voters, with Mr Buttigieg up by 52 per cent to 41 per cent, and Ms Harris ahead by 51 per cent to 42 per cent.

The poll reflects national findings; other surveys in the states expected to play an outsized role in the 2020 contest have shown tighter races, particularly in parts of the upper midwest that Mr Trump flipped from Democrats in 2016.

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Nonetheless, the national results represent a shift away from Mr Trump since the summer, when only Mr Biden had a clear advantage over the incumbent. With Republicans and Democrats mostly locked into their voting intentions, the biggest difference between the results from July and those in the new poll is that independents have moved in the direction of the Democratic candidates.

One wild card is any possible impact that the impeachment proceedings might have on public attitudes. The current poll was completed just before the House voted along party lines to formalise the inquiry.

As the process moves to public hearings in the House and what many now expect to be articles of impeachment against the president and then a trial in the Senate, both sides will be watching for cracks in either Mr Trump's or his challengers' coalitions.

National margins of the size in the new survey, if they were to hold for another year, would probably result in a popular-vote victory for the Democrats, possibly larger than the nearly 3 million-vote margin that Hillary Clinton earned over Mr Trump in 2016.

That means Mr Trump's path to victory next year is to replicate the electoral college majority he fashioned by narrowly winning Florida, North Carolina and three states that had long been Democratic presidential strongholds - Pennsylvania, Michigan and Wisconsin - while easily winning Ohio and Iowa, which had been swing states in recent elections.

State-specific polls often have shown Mr Trump's job approval rating higher than it is nationally, pointing to the challenges for Democrats in their efforts to win back the White House next year. Though the president is not in a comfortable position heading into the election, Democrats still could have limited options and some obstacles to winning an electoral college majority regardless of the popular vote.

Mr Trump runs behind the Democratic candidates despite generally positive assessments of the economy. A plurality (44 per cent) of Americans say the economy has gotten better since Mr Trump took office, twice the number who say it has gotten worse (22 per cent). About a third say the economy has stayed about the same.

Of those who say it has gotten better, more than 3 in 4 give Mr Trump a great deal or a good amount of credit. Of those who say it has gotten worse, more than 8 in 10 say Mr Trump deserves the blame.

Offsetting the benefits of the economy, Mr Trump gets negative views on a series of attributes. Just over 3 in 10, or 31 per cent, say he is honest and trustworthy, tying the lowest result of his presidency. A slightly larger 36 per cent say he has the kind of personality and temperament it takes to serve effectively as president. In both cases, just under 2 in 3 voters disagreed. Those findings are almost identical to results from a survey in October 2016, shortly before he won the presidency.

On other questions, 37 per cent say Mr Trump understands the problems “of people like you,” 40 per cent say he is good at making political deals, and 42 per cent say he has brought needed change to Washington.

Overall, these results are low for an incumbent president, but Mr Trump proved that he could overcome them in 2016 and his allies think he can do so again in 2020.

Thus far in the campaign, Mr Trump has worked to enthuse his past supporters rather than try to extend his reach beyond already-loyal elements of the electorate. But the poll shows he is faring worse now than in 2016 election polls among white voters without a university degree as well as political independents, both groups that helped power his victory. In 2016, he won among white voters without a degree by a 36-point margin, according to a Pew Research Center analysis, but he leads Mr Biden by half that - 18 points - among registered voters in this group in the latest Post-ABC poll.

Mr Trump narrowly won self-identified independents in 2016 (46 per cent to 42 per cent) according to the National Election Pool exit poll, but in the latest Post-ABC poll he trails Mr Biden by 17 points among this group.

Mr Trump also receives less unified support among Republican voters - 80 per cent in the current poll compared with 88 per cent in the 2016 exit poll. Based on recent elections, Mr Trump would expect to gain support among Republicans through the course of a re-election campaign.

As other markers, including fundraising and activism, have shown, voter intensity is high one year out from the election. About 8 in 10 adults say they will definitely vote in the 2020 election, and it appears to make little difference whether the Democratic nominee is Mr Biden, Ms Warren or Mr Sanders. Similarly large majorities of Democrats and Republicans say they will definitely vote, regardless of the Democratic challenger.

But people ages 18 to 29 are slightly more likely to say they could skip the vote if the nominee were Mr Biden or Ms Warren - 34 per cent and 38 per cent, respectively - than if the nominee is Sanders (22 per cent). Voters in other age groups say they are less likely to skip voting next year.

Mr Biden builds his national margin over Mr Trump with the support of female voters. He and Mr Trump run evenly among men, at 47 per cent. But Mr Biden is swamping the president among women, 64 per cent to 33 per cent. Every Democrat tested gets at least 59 per cent among women.

Among whites, all five Democrats are winning among those with college degrees, helping to blunt Mr Trump's advantage among those without college degrees. Among non-white voters, the Democratic candidates are running far ahead of the president.

The Post-ABC poll was conducted by telephone between 27-30 October among a random national sample of 1,003 adults, 65 per cent of whom were reached on cellphones and 35 per cent on landlines. The margin of sampling error for results among adults is plus or minus 3.5 percentage points; the error margin is four points among the sample of 876 registered voters.