LONDON — Former British Treasury chief Rishi Sunak worked to stave off momentum from challenger Penny Mordaunt in the race to replace Prime Minister Boris Johnson, with Conservative Party lawmakers set to knock one of the six remaining contenders out of the contest on Thursday.

Sunak, who quit as Britain’s Treasury chief last week, got the most votes in a first-round ballot on Wednesday, with junior trade minister Mordaunt a strong second. Bookmaker Ladbrokes said Mordaunt was now the favorite to win the leadership election, followed by Sunak and Foreign Secretary Liz Truss.

Whoever wins, the contest will replace the flamboyant, flawed Johnson with a new and much lesser-known prime minister. Only Conservative lawmakers and party members have a say in the vote to choose Johnson’s successor.

Truss placed third on Wednesday and is trying consolidate support from lawmakers on the party’s right, who mistrust Sunak’s high spending on support to people and businesses during the coronavirus pandemic, and the tax increases he brought in as COVID-19 hammered Britain’s economy.

In a campaign launch speech, Truss cited her international experience mustering support for Ukraine in response to Russia’s invasion and striking deals with other countries in her former role as trade secretary.

She said she would set the British economy on an “upward trajectory” by 2024, which is the deadline for the next national election.

Sunak argues that the immediate tax cuts promised by his rivals are reckless amid economic shockwaves from the pandemic and the war in Ukraine.

He said his “number one economic priority is to tackle inflation” — forecast to hit 11% later this year — before cutting taxes.

Sunak also has faced allegations he is out of touch with ordinary people’s struggles because of his wealth. He is a former investment banker, and his wife is the daughter of the billionaire founder of Indian tech company Infosys.

“I don’t judge people by their bank accounts, I judge them by their characters,” Sunak told the BBC. “And I think people can judge me by my actions over the past couple of years.”

Three other candidates remain in the race. Lawmakers will vote again Thursday, and the lowest-placed contender will drop out.

Further rounds of voting are due to take place next week until just two candidates remain.

The final two contenders will face a runoff vote by about 180,000 Conservative Party members across the country. The winner is scheduled to be announced Sept. 5 and will automatically become prime minister, without the need for a national election.

The contest was triggered when Johnson resigned as Tory leader last week after his party revolted after months of ethics scandals and government ministers began resigning en masse. He will remain in office as a caretaker prime minister until his replacement as party chief is chosen.

Truss had been seen as a front-runner, but gained fewer votes Wednesday than Mordaunt, who scores highly in polls of party members.

Unlike Sunak and Truss, Mordaunt didn’t hold a senior post in Johnson’s government, though she was a junior minister. An affable politician from a military family, she is widely seen as a breath of fresh air and has been scoring highly in polls of party members.

Despite pleas by party officials to keep the campaign upbeat, supporters of the respective contenders are busily attacking the experience and competence of the competition.

Former Brexit negotiator David Frost, a hard-line euroskeptic and an ally of Truss, said Mordaunt could not be trusted to maintain the firm Brexit conditions that his wing of the party favors.

He said that when Mordaunt was his deputy in Brexit negotiations, “she wouldn’t always deliver tough messages to the European Union when that was necessary.”