Bill Clinton is expected to take a break from his wife’s campaign on Friday night – just days before she faces crucial ­primary contests in Texas and Ohio – to host a charity event in Toronto with Frank Giustra, a Canadian ­businessman.

The former president’s relationship with Mr Giustra, a mining tycoon and ­philanthropist who has donated more than $100m to Mr Clinton’s foundation, has come under scrutiny in recent weeks. It has been suggested that his ties to Mr Clinton have helped Mr Giustra’s business ventures, including a lucrative mining agreement in Kazakhstan.