Michael Gove was battling to stay in the Tory leadership contest after his admission of cocaine use led to calls to withdraw.
The Environment Secretary will attempt to present himself as a “serious” alternative to the contest’s frontrunner Boris Johnson when he launches his campaign on Monday and insists he is “undaunted” by criticism.
But he suffered a blow as Work and Pensions Secretary Amber Rudd declared her support for his rival Jeremy Hunt.
And he faced calls to pull out of the contest for Number 10 by former Tory chairwoman Baroness Warsi.
Former foreign secretary Mr Johnson is the bookmakers’ favourite to replace Theresa May and the main focus at Westminster is on who will challenge him when the candidates are reduced to a final two next week.
Mr Hunt and Mr Gove, the MPs seen as his most likely competitors, both launch their campaigns with Mr Johnson fixed in their sights.
Mr Gove, whose campaign plans have been derailed by the cocaine furore, will attempt to persuade Tory MPs that he can be the “serious leader” the country needs.
The Cabinet minister, who endured a bruising interview on BBC’s Andrew Marr Show where he was repeatedly questioned about his cocaine use 20 years ago, will attempt to portray his willingness to face scrutiny as a strength.
Critics of Mr Johnson have highlighted his lack of media appearances since Mrs May announced her decision to quit.
At his launch event, Mr Gove will say: “I have led from the front undaunted by criticism and resolute in the need to solve complex issues because that is what our country needs.”
In a message to Tory MPs, he will add: “We need a leader who is ready to lead from day one.
“A leader ready to be prime minister from day one.
“A leader ready to face the scrutiny of the studio lights.”
Mr Gove will say he can both deliver Brexit and “stop Jeremy Corbyn ever getting the keys to Downing Street”.
But Lady Warsi said it was “completely inappropriate” for him to remain in the contest to be the next prime minister.
She told Channel 4 News: “This case isn’t just about drug taking, it is about trust, it is about hypocrisy of the highest order and it cannot be that we have somebody who is now mired in this issue of trust and hypocrisy feel that it is still appropriate for him to stand as leader of the Conservative Party and a prime minister of this country.”
Mr Gove’s campaign suffered a further setback when Ms Rudd, an influential voice on the Remain-supporting wing of the party and leader of the centrist One Nation group, came out in support of Mr Hunt.
She said: “These are serious times and we need a respected statesman who Brussels will listen to, not more bluster.”
Foreign Secretary Mr Hunt will use his launch event to stress his credentials as a statesman who can handle the complexities of Brexit.
“We are facing a constitutional crisis. Our new prime minister will preside over a hung Parliament,” Mr Hunt will say.
“This extremely serious moment calls for an experienced, serious leader. We need the art of tough negotiation, not the art of empty rhetoric.”
The first stage of the leadership contest takes place on Monday with candidates required to gather support from eight fellow Tory MPs by 5pm to enter the race.
As well as Mr Gove and Mr Hunt, Health Secretary Matt Hancock and former Brexit secretary Dominic Raab are holding events to launch their campaigns.
Mr Hancock will say he offers the “fresh start” the country needs and will set out his vision to make the next decade “the soaring ’20s” for the post-Brexit UK.
Mr Raab will unveil a package of proposals to develop clean energy and protect the environment – including redirecting £500 million a year from the aid budget to create an international wildlife fund to save endangered species and habitats.
“We’ve got to look to the future,” he will say. “We’ve got to leave the environment in a better state than we found it.”
Meanwhile Mr Johnson used his Daily Telegraph column to signal a plan to slash the higher rate of income tax for people earning more than £50,000.
“We should be raising thresholds of income tax – so that we help the huge numbers that have been captured in the higher rate by fiscal drag,” he said.
Home Secretary Sajid Javid picked up further support for his campaign, with ministers Caroline Nokes and Victoria Atkins backing him.