Hillary Clinton's chances of becoming president suffered a double blow as a new national poll put her in a dead heat with Donald Trump, and large numbers of Bernie Sanders supporters indicated they would switch to the Republican billionaire.

A Reuters/Ipsos survey put Mrs Clinton on 41 per cent to Mr Trump's 40 per cent, a statistical tie, and showed a huge surge in support for the property mogul who was 13 points behind in the same regular poll a week earlier.

Following Tuesday's Democratic primary in West Virginia exit polls showed 39 per cent of those who voted for Mr Sanders, a self-declared socialist, would back Mr Trump in a general election contest between him and Mrs Clinton.