The race to succeed Liz Truss as Britain’s prime minister was already gathering pace on Friday morning, potentially pitting the former prime minister, Boris Johnson, against the man who helped oust him from Downing Street just a little more than three months ago, the former finance minister, Rishi Sunak.
Penny Mordaunt, now a senior minister, was also seen as a serious potential contender in an election that will be conducted within the governing Conservative Party, which controls the government and can select a prime minister without calling a general election.
The next government leader faces a formidable task as Britain heads into an economic slowdown with inflation surging, borrowing costs rising and a winter likely to be dominated by labor strikes and worries about energy supplies.
Ahead of what could be a head-on battle against Mr. Johnson, Mr. Sunak’s supporters are presenting him as the safe pair of hands, the man who can restore stability following the crisis precipitated by Ms. Truss’s government when it announced unfunded tax cuts last month, sending financial markets into a tailspin. The British pound plummeted and borrowing costs soared.
Less than seven weeks after she took office, Ms. Truss resigned on Thursday — the shortest-serving prime minister in British history.
This summer, after Mr. Johnson was forced to resign, Mr. Sunak ran to succeed him but lost out to Ms. Truss. During that leadership contest he gave a prophetic warning of the risks of her economic program, including the tax cuts that ended up rattling the markets.
So the appointment of Mr. Sunak, an experienced former chancellor of the Exchequer, might reassure financial markets enough to give a new government more leeway when it comes up with a new budget plan.
“Rishi is the experienced leader to sort the economy, lead effectively, get us back into political contention and unite the Party and country,” wrote Bim Afolami, a Conservative lawmaker, on Twitter.
But the prospect of an extraordinary return for Mr. Johnson, who left Downing Street less than two months ago under a sizable cloud, has galvanized the looming contest.
Reinstalling him would be a risk for the Conservative Party because he quit after a succession of ethics scandals and is still being investigated by a parliamentary committee over claims he misled lawmakers about lockdown-busting parties at his Downing Street office and residence.
Mr. Johnson has been on a Caribbean vacation. But his father, Stanley Johnson, did little to dispel the impression that his son was preparing a comeback attempt. He told Britain’s ITV television network on Friday: “I think he’s on a plane, as I understand it.”
By the time he left office, Mr. Johnson, always a polarizing figure, was deeply unpopular with voters, according to opinion polls. He tarnished his party’s reputation and dozens of members of his government resigned.
But since then, the Conservative Party’s support has collapsed. A new opinion poll showed the party plunging to a new low in support of just 14 percent.
Mr. Johnson’s supporters argue that because he won a landslide election victory in 2019, he has a mandate from the voters, and his brand of optimism could help rally the Conservatives.
If he were to run, he would be seen as the clear favorite as he remains popular among party members who could make the ultimate decision.
It was they who, during the summer, rejected Mr. Sunak in favor of Ms. Truss when the two reached the final stages of the last leadership contest. One of the main reasons for his failure was the perception among party members that Mr. Sunak betrayed Mr. Johnson by resigning from his cabinet, prompting the crisis that destroyed his leadership.
Even before formal declarations by any candidates, their allies were canvassing lawmakers, who in some cases offered their public support.
Among those urging Mr. Johnson to run are Jacob Rees-Mogg, the business secretary, who wrote on Twitter that he was supporting the former prime minister under the hashtag #BORISorBUST. Nadine Dorries, another strong supporter in Parliament, described him as a “known winner.”
More centrist lawmakers might also be tempted to support Mr. Johnson because of his history of success in elections before his recent ethics scandals. But his critics say he would struggle to unite his colleagues, and one Conservative lawmaker, Roger Gale, has said he would resign from the party if Mr. Johnson returned.
Ms. Mordaunt, who finished third in the summer leadership contest, has good communication skills and has raised her profile in recent weeks, including this week when she appeared in Parliament to defend the government.
She is relatively untested at the highest reaches of government. But her supporters argue that she has more experience than former prime ministers like Tony Blair and David Cameron who never held ministerial positions before taking power, having been in opposition.
And Ms. Mordaunt might be the best placed of the possible contenders to manage a fractured Conservative Party.
“Penny is the best candidate to unite our party and lead our great nation,” said Bob Seely, one of her supporters and a member of Parliament.
None of the likely front-runners have yet declared, but by Monday afternoon, contenders must have nominations from at least 100 of the 357 Conservative lawmakers, a number intended to speed up the contest by limiting potential candidates to a maximum of three.
If only one politician passes that threshold, he or she will become prime minister on Monday. If there are two or three, Conservative lawmakers will vote on Monday and the top two will then go for a final decision on Oct. 28 in a vote by about 170,000 Conservative Party members — unless one withdraws voluntarily.
Other possible candidates include the home secretary, Suella Braverman, who was fired by Ms. Truss on Wednesday, and Kemi Badenoch, the international trade secretary. However, on the evidence of their performance in the summer leadership contest, neither are seen as likely to reach the threshold of 100 nominations.
On Friday, the defense secretary, Ben Wallace, ruled himself out of the contest, and said he was leaning toward support for Mr. Johnson.
