Bernie Sanders at a rally in Santa Ana, California. Some moderates fear that if he wins big on Super Tuesday next week, when 14 states hold their Democratic primaries, he will be unstoppable

Concerns among moderate Democrats about Bernie Sanders’s electability grew yesterday when he admitted that he had no idea how much his proposals would cost, even as he added universal childcare to his range of policies.

Mr Sanders, 78, who has become the frontrunner in the Democratic race to take on President Trump after a big win in the Nevada caucus at the weekend, told CBS that childcare provision, costed at $1.5 trillion over a decade, would be covered by a “tax on wealth”.

During the interview Mr Sanders said he supported some of the socialist policies of Fidel Castro, the Cuban revolutionary leader. His insistence that it was unfair to dismiss everything about the Castro regime was judged by Democrats in Florida to have killed