“Mr Sunak’s problem, of course, is that Remain voters and Hunt supporters are very much in the minority among Tory members,” he told The Telegraph.
“Wherever he currently stands in the race, his challenge this week is to strengthen his appeal to the mainstream of his party.”
While two-thirds of the former chancellor’s supporters backed the decision to sack the prime minister, two-thirds of those planning to vote for Mrs Truss opposed it.
“In this context, it is not surprising that Tory members believe that Truss will be better able to handle the various issues facing the government and is more likely to help the party win a general election,” said Sir John in The Times.
Sunak has repeatedly insisted that he was not disloyal to the prime minister, arguing that he served his cabinet faithfully for two and a half years.
He has said he resigned after it became clear he and Johnson had fundamentally different views on the economy and how to manage public spending.
“There is nothing disloyal about leaving in this circumstance. It is the right thing to do for the country because the people deserve to have these two people together,” he said in Exeter.
“I hope Boris ends up on the ballot again”
Mr Sunak was accused of backstabbing the Prime Minister by Matthew Dransfield, a Tory member, at the first leadership meetings in Leeds last week.
Afterwards, the 47-year-old business consultant from West Yorkshire told The Telegraph that Johnson had been the victim of a “coup”.
He insisted he would not be voting for the former chancellor but could back Ms Truss, adding he still hoped the prime minister could stay in office.
“I hope Boris ends up on the ballot again,” he said, describing the prime minister as “a cut above” the two candidates in the race.
He added that the role of Mr. Sunak in the fall of Mr. Johnson should effectively exclude him from the leadership, as “I don’t think it is possible to get over this act that he has committed.”
Dransfield added: “I think it’s treacherous. It’s a terrible thing to do and it’s not a good thing for the country at the moment. Boris Johnson won a landslide victory.”