LONDON — Early Sunday, former British finance minister Rishi Sunak announced his bid to replace Liz Truss as the next leader of the Conservative Party and prime minister of Britain. By nightfall, his victory appeared close, albeit not certain.

In the intervening hours, his most serious rival all day, former prime minister Boris Johnson, said that he would not seek to return to the job from which he was ousted in July. Johnson, who never formally announced his bid, had attempted unsuccessfully to round up sufficient support from Conservative members of Parliament. Penny Mourdant, the leader of the House of Commons, made a run for the leadership post but had not come close to either Sunak or Johnson on Sunday, though she could pick up some of Johnson's supporters.

A fast-paced leadership contest in Britain is unpredictable sport. Barring a major development, however, Sunak conceivably could be selected as early as Monday afternoon. If ultimately victorious, Sunak would become the country's first prime minister of South Asian descent or of color. He was born in Southampton, England, to parents of Indian origin who emigrated from East Africa.

"I want to fix our economy, unite our Party and deliver for our country," Sunak said in his Sunday tweet.

It is the second time in less than four months that Sunak, 42, has vied for the role. Over the summer, the former chancellor of the exchequer made it to the final round in the race to succeed Johnson before losing to Truss when the vote was put to party members.

A relative unknown before his promotion in 2020 to be Johnson's finance minister, Sunak has since developed a reputation for attention to fiscal detail and taking a pragmatic approach to managing the country's finances, an image he established by steering the country through the economic chaos unleashed by the coronavirus pandemic. He and Johnson became something of a contrasting double act at the top of British government: While the prime minister appealed to the public with his unique humor, Sunak played the role of the sensible accountant in the room.

Sunday's announcement made Sunak the first — and, so far, the only — formally declared candidate to have collected the 100 nominations from fellow lawmakers required by 2 p.m. Monday to appear on the party's ballot, according to public tallies. If more than one candidate passes the threshold, members of Parliament will select two to be put to an online vote by party members, with the results expected Oct. 28.

As of Sunday morning, Sunak's strongest challenger appeared to be Johnson, the former prime minister whose resignation in July kicked off Britain's current bout of political chaos. In a tweet containing his formal letter of resignation as Johnson's finance minister, which prompted a wave of others to quit and ultimately forced Johnson to resign, Sunak said the public deserved a government that conducted itself "properly, competently and seriously."

On Saturday, reports in the British media said the two men — who once worked side by side — were holding late-night talks, prompting speculation that the two could strike a deal to put their rivalry aside.

A number of Conservative lawmakers and former Johnson allies, including former cabinet members Sajid Javid and Gavin Williamson, have announced their support for Sunak.

In a significant coup for the Sunak camp, Suella Braverman — a rising star within the party's right-wing whose resignation as home secretary last week catalyzed Truss's resignation — also threw her weight behind the former finance minister, arguing that he offers stability amid the "dire straits" in which the country currently finds itself.

"I need a leader who will put our house in order and apply a steady, careful hand on the tiller. That person, for me, is Rishi Sunak," she wrote in the Telegraph newspaper Sunday.

David Frost — who was responsible for negotiating the Brexit deal and later given a seat in the House of Lords by Johnson - declared on Saturday that it was time to "move on" from the former prime minister.

Many of Sunak's backers seek to portray him as a stabilizing candidate who is capable of bringing the chaos of recent months to an end. Sunak loyalists also pointed out that during the leadership contest against Truss this summer, his candidacy received the most support from his parliamentary colleagues.

However, critics within the Conservative Party worry that he is out of touch with voters and have accused him of disloyalty to Johnson — a key source of contention for many of the party's grass-roots members among whom Johnson remains popular.

Educated at Winchester, one of Britain's most prestigious private schools Sunak has a glittering résumé, with degrees from the University of Oxford and Stanford University and a stint at Goldman Sachs. One of the wealthiest British politicians, he is married to the Indian tech heiress Akshata Murthy, whose tax status caused Sunak some political discomfort during his leadership campaign in the summer.

And a video clip from a 2007 BBC documentary, in which Sunak suggests he doesn't have any "working-class friends," is recirculating online as some Britons frown upon the array of upper-class Conservative contenders.

Nonetheless, Sunak remains popular among Conservative politicians, although he fares less well among the party's national membership, which favored Truss in September by 57.4 percent to Sunak's 42.6 percent.

To his supporters, Sunak is a steady hand on the economic tiller, as he correctly predicted the market crisis sparked by Truss's policies when she slashed taxes and sent the British pound plummeting. He called Truss's proposed economic reforms "fairy tale" economics before she took office, an assessment that is likely to lend credence to his image of fiscal responsibility.

A blot on his record, however, is his link to the "Partygate" scandal that toppled Johnson's government. Like his boss, Sunak was fined by London's Metropolitan Police while in office for attending gatherings at 10 Downing Street while Britons were under severe government-imposed coronavirus lockdown restrictions. And some critics, like former Conservative Party leader Iain Duncan Smith, have noted that the U.K.'s record-high levels of inflation began during Sunak's time as chancellor.