Bernie Sanders was on the cusp of being declared the winner of the Alaska caucus when the email from Robby Mook, Hillary Clinton’s boyish campaign manager, arrived.

It was short and to the point. The Clinton campaign was being outspent by thousands upon thousands of supporters chipping in a few dollars here and there for the insurgent self-declared Socialist senator from Vermont. In just three paragraphs the message combined optimism with irritation that he was still in the contest.

Mrs Clinton may still enjoy a healthy lead among the delegates who will choose the Democratic presidential nominee at the party’s convention in Philadelphia in late July, but she is finding Mr Sanders impossible to shake off – and he has made it pretty clear he is not going away any time soon.