The trade minister has joined forces with Ms Truss just weeks after she was knocked into third place by the Foreign Secretary.
Ms Mordaunt finished just eight votes behind Ms Truss in the ballot among Tory MPs, but her endorsement is a major coup for the 47-year-old and a significant blow for former Chancellor Rishi Sunak.
The MP for Portsmouth North said: “I could have remained undeclared, I could be off sipping pina coladas right now but I’m not because this is too important and I’m not going to leave this to chance.”
Referring to Ms Truss, she added: “Her graft, her authenticity, her determination, her ambition for this country, her consistency and sense of duty – she knows what she believes in, and her resolve to stand up against tyranny and fight for freedom.
“That’s what our country stands for and that’s why I know with her we can win.”
Penny Mordaunt ran for the Conservative Party leadership before endorsing Liz Truss
Victoria Jones
Foreign Secretary Liz Truss is hoping to be the next Prime Minister
Finnbarr Webster
Ms Mordaunt concluded: “She, for me, is the hope candidate.”
It came on stage during the latest hustings event in Exeter on Monday evening.
Opinion polls suggest Ms Truss enjoys a significant lead over Mr Sunak among Tory members, who will decide the next prime minister.
In Exeter, Ms Truss sought to appeal to members by promising a “war on Whitehall waste”, cutting civil service time off, ending national pay deals and scrapping jobs aimed at increasing inclusion and diversity in the public sector.
She claimed her plans would save £11billion and tackle left-wing “groupthink”.
Both candidates appeared on stage separately during the hustings event in Exeter, in what proved to be a calmer affair than the head-to-head format of the TV debates.
Ms Truss branded the Treasury a “block on progress” and hinted at making changes to the way it operates if she were to become prime minister.
But Mr Sunak hit back at her economic plans, to noticeable applause from the party members in attendance.
He said: “I want to reform the corporate tax system … Liz Truss’s policies on corporate taxation are exactly the failed Treasury orthodoxy of the last 10 years, which hasn’t worked.
"I want to change it and grow the economy.”
Mr Sunak outlined a greater role for technology in reforming the NHS, explaining: “Right now we’ve got this massive problem with the backlogs and, as they’ve already done in some trusts, we can use software automation and AI to massively rip through all of the triaging and processing of them and clear up the lists, and that’s going to clear up processing times.”
While the tone of the hustings was largely devoid of blue-on-blue attacks, Ms Truss criticised “attention seeker” Nicola Sturgeon.
She said of Scotland’s First Minister: “I think the best thing to do with Nicola Sturgeon is ignore her.”