“Republicans this year in particular have an anti-establishment, anti-politician orientation,” says Larry Sabato of the University of Virginia. “And no name screams louder [establishment] than Bush.”

Mr Bush and his supporters have raised more than $100m and he remains the favourite of the GOP elite. But he has failed to excite the grassroots who view his conservatism with suspicion. He has appeared wooden on the campaign trail and has been unexpectedly gaffe-prone.

Mr Trump pounced on his recent “clumsy move” to defend the use of the phrase “anchor babies” — a derogatory term for children born in the US to non-citizens — by saying it applied more to Asians than Hispanics.

“Jeb is just boring,” says one GOP official who knows Mr Bush. “Trump has been nailing him and he hasn’t fought back.”