A Briton who tried to grab a police officer's gun at a Donald Trump rally in Las Vegas says he wanted to shoot the US candidate, court papers say.

Leading Republicans united Tuesday in an extraordinary denunciation of Donald Trump's attacks on a federal judge, with House Speaker Paul Ryan calling them the "textbook definition of a racist comment." Yet Ryan stood by his endorsement of the presumptive presidential nominee.

Renowned theoretical physicist Stephen Hawking may have a good grasp of the workings of the universe, but he says he can not understand Donald Trump's popularity.

Donald Trump wins presidential primaries in all five north-eastern US states voting while Hillary Clinton takes four.

The Democratic primaries have been remarkably free of rancour, thanks to the mostly civil and substantive political debate. The Republican primaries, on the other hand, have become a three-ring circus.

US presidential candidates are making a final pitch to people in the state of Iowa, where the first votes for party nominations will take place on Monday.