The former chancellor will insist he has a plan to deal with the economic challenges the country is facing, saying it is a matter of when, not if the tax burden starts to fall.
At an event to launch his campaign on Tuesday he will receive heavyweight support from another former chancellor, Lord Norman Lamont, who said Mr Sunak had the courage to make the “tough decisions” to deal the “extremely serious” economic situation.
Mr Sunak is alone among the contenders to succeed Boris Johnson in not promising immediate tax cuts if he wins the race to become Tory leader.
He has come under attack from allies of the prime minister who believe his announcement last week that he was quitting helped to start the rush of resignations that forced Mr Johnson to admit his time was up.
But in his address, Mr Sunak will seek to make a virtue of his willingness to confront difficult economic realities.
“We need a return to traditional Conservative economic values, and that means honesty and responsibility, not fairy tales,” he will say.
“I have had to make some of the most difficult choices in my life when I was chancellor, in particular how to deal with our debt and borrowing after Covid.
“I have never hidden away from those and I certainly won’t pretend now that the choices I made, and the things I voted for, were somehow not necessary.
"Whilst this may be politically inconvenient, it is the truth.”
“My message to the party and the country is simple: I have a plan to steer our country through these headwinds. Once we have gripped inflation, I will get the tax burden down. It is a question of ‘when’, not ‘if’.”
Before the launch event in central London, Lord Lamont said: “The country faces an extremely serious economic situation.
“To weather the storm requires a high degree of competence, matched by the courage to make really tough decisions. The public understand this better than many politicians and will respond.
“Tax cuts unmatched by spending cuts achieve nothing. Yes, the tax burden needs to be reduced, as Rishi also believes, but only as and when the public finances allow.
“Mrs [former prime minister Margaret] Thatcher often said dealing with the deficit comes even before reducing taxes. Deficits are just delayed taxation.
“Rishi has the skill, determination and ideas to get us through this difficult period into more prosperous times.”