The foreign secretary and former chancellor are the last two standing following a series of rounds of voting by Conservative members of Parliament.
Now the contest moves to the party grassroots. Over the course of August, just under 200,000 Tory members will vote.
On Sept. 5, the U.K.’s new prime minister will be revealed.
If the past two weeks are anything to go by, there is little love lost between the two rivals and the contest is expected be rough.
Sunak may have won the support of most MPs, but Truss will likely be confident she can win over the membership.
Truss probably now goes into the contest as the favorite, having completely reinvented herself politically over the past few years.
The 46-year-old was first elected in 2010 having been placed on David Cameron’s “A-list” of candidates, designed to help him modernize the party.
She has served in the cabinet since 2014, under Cameron, Theresa May and Boris Johnson.
Truss has been environment secretary, justice secretary, chief secretary to the Treasury, trade secretary and now foreign secretary.
A Liberal Democrat in her university days, Truss was also a prominent Remain campaigner in the referendum on leaving the European Union in 2016.
But with the zeal of a convert, she has successfully repositioned herself as an arch-Brexiteer.
A key plank of her leadership campaign is also to “start cutting taxes from day one” with a new budget.
It is a dividing line with Rishi Sunak that could prove popular with the party membership.
Truss also did not resign from cabinet as ministers deserted Johnson, winning the support of prominent allies of the outgoing prime minister including Nadine Dorries and Jacob Rees-Mogg.
In her current job at the Foreign Office, Truss has been hawkish about the need for the West to stand up to Russia over its invasion of Ukraine.
But she has also been criticized for mis-steps, including her support for Britons who wanted to fly to the war-torn country and over what some have branded her “Instagram diplomacy.”
Possibly aware of her past reputation for goofy interventions, including a viral speeches about “pork markets” and cheese imports, she has moved to stiffen her public persona.
The former chancellor formally known as “dishy Rishi” had long been seen as the most likely to succeed Johnson, but he has been bruised by a series of missteps and events.
He has pitched himself as the fiscally conservative candidate, attacking Truss for a “fairytale” plan to immediately cut taxes and borrow more.
The 42-year-old has promised to cut taxes but to do so in a “way that’s responsible” and only “after we’ve got a grip of inflation.”
His dramatic resignation from cabinet two weeks ago, while Johnson was painfully explaining to the BBC why he had given a job to a Tory MP he knew had been investigated for harassment, helped trigger the prime minister’s downfall.
Johnson has not endorsed anyone in the race, but the plea during his final prime minister’s questions for his successor to “cut taxes” will be interpreted as implicit backing for Truss.
Having handed out billions during the pandemic alongside slick personal PR, Sunak’s ascent seemed almost unstoppable.
But the cost of living crisis as well as revelations about his wife’s tax arrangements, and the fact he held a U.S. green card while serving in government, damaged his brand.
Like Johnson, Sunak was also fined by police for breaking COVID rules as he was present at the birthday gathering for Johnson in the Cabinet Room.
His spring statement mini-budget in March triggered a huge backlash, with critics warning it did not do enough to help people struggling with the cost of living.
Sunak moved to address those criticisms in May, with a £21 billion package of measures designed to help people pay for energy bills.
While this pleased some in his party, it angered those who are instinctively against higher taxes and more borrowing.
If the core of Truss’ campaign is likely to be immediate tax cuts, Sunak’s can be expected to argue he has wide national appeal and therefore the better chance of beating Labour at the next general election.