Donald Trump is escalating the cold war with his own party leadership, using campaign rallies and Twitter to chastise Mitch McConnell, the most senior Republican in the Senate, and hurling insults at senators.

Senior party figures fear his rhetoric risks effectively eliminating his two-seat majority in the Senate and say the rift could not come at a worse time.

Not only do they want to begin work on tax reform, but next month Congress must pass legislation to keep funding the government and to raise the federal borrowing limit. Failure could prompt a government shut-down or financial collapse.

On Tuesday Mr Trump used a campaign rally in Arizona to accuse one of the state’s senators of being “weak” on immigration. After promising not to use his name, he repeated the jibe on Twitter yesterday and named Jeff Flake.