A Republican state senator from Iowa has become the first elected official to boycott his own party in protest at Donald Trump's "racist remarks and judicial jihad".

David Johnson, a Republican state senator from Iowa, compared the rise of Mr Trump, the presumptive Republican nominee for president, to Adolf Hitler in 1930s Germany.

His decision to leave the party was prompted by Mr Trump's claims that Gonzalo Curiel, a federal judge who is presiding over the Trump University lawsuit, was biased because of his Mexican heritage. Mr Curial was born and raised in America.