Support for US Republican presidential candidate Donald Trump has plummeted to 39 per cent, a new poll shows.

The Washington Post-ABC News poll on Sunday showed the dramatic decline from 46 per cent last month.

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The same poll showed support for his opponent, Hillary Clinton, the presumptive Democratic nominee, increase from 44 per cent last month to 51 per cent this month, giving Clinton her largest lead in the Washington Post-ABC poll since late last year.

Roughly two out of three Americans say they think Trump is unqualified to lead the nation, according to the poll.

Similar majorities said the idea of him as president makes them anxious and his comments about women, minorities and Muslims show an unfair bias.

But the poll also showed Clinton’s weaknesses, underscoring what the Post said was the “historic unpopularity” of the two major-party candidates.

A smaller but still significant majority of people questioned they disapproved of the way Clinton has handled questions about her use of a personal email server while she was secretary of state.

Half of the people surveyed said they were anxious about the prospect of a Clinton presidency.

The survey polled 1001 people from June 20-23.

It had a margin of error of 3.5 per cent.