Barack Obama on Thursday said he had raised almost $8m in less than 48 hours since Super Tuesday, which was almost double Mrs Clinton’s tally in what is turning into the most frantic spell of fundraising since the US presidential contest began.

The two Democratic candidates, who fought each other to a standstill in Tuesday’s mega-primary, now face a punishing schedule of primaries in the 22 states that have not yet participated in what is the closest-fought and longest-running nomination battle in memory.