LONDON — Exhausted rather than exuberant, the two candidates to succeed Prime Minister Boris Johnson held their last scheduled joint campaign appearance on Wednesday, trading final jabs in a contest that has often seemed to have little to do with the economic storm clouds gathering over Britain.
The underdog, Rishi Sunak, claimed that his economic proposals would bring down Britain’s runaway inflation faster than those of his opponent, Liz Truss. He suggested that Ms. Truss’s promise of tax cuts, without parallel spending cuts, would jeopardize the country’s reputation in global credit markets.
Ms. Truss, who currently serves as foreign secretary, said that when Mr. Sunak was chancellor of the Exchequer, he had been wrong to push tax increases. She ruled out another windfall profits tax on energy companies after Mr. Sunak imposed a tax worth 5 billion pounds ($5.8 billion) on North Sea oil and gas companies last May.
The winner will be announced on Monday, chosen in balloting not by tens of millions of British voters, but by roughly 160,000 dues-paying members of the Conservative Party. With the party holding a majority in Parliament, its new leader will become prime minister.
Neither candidate has offered a comprehensive package to deal with families hard hit by spiraling food and fuel prices. Mr. Sunak has proposed cutting the value added tax on energy bills, while Ms. Truss has promised targeted aid to consumers.
With Ms. Truss’s emphasis on tax cuts, the campaign has seemed increasingly untethered from the reality of a collapsing British economy. Household energy bills recently shot up by 80 percent; Goldman Sachs warned that inflation could reach 22 percent early next year; and the Bank of England forecast a lengthy recession.
The last joint appearance by the rivals, at Wembley Arena in London, marked the end of a contest that has seemed both grinding and oddly static, stretching back to July 20, when Conservative lawmakers winnowed a field of 11 candidates to two finalists.
Ms. Truss has held a commanding lead in the polls over Mr. Sunak through 12 party rallies similar to Wednesday’s. Despite Mr. Sunak’s best efforts to shake up the race, Ms. Truss has kept an iron grip on her front-runner status, preaching a message of lower taxes and smaller government to a receptive audience of faithful Tories.
Mr. Sunak has presented himself as the candidate of hard truths, warning party members that the government cannot afford to cut taxes before it tames the country’s double-digit inflation.
While his command of the facts and smooth speaking style drew more applause than Ms. Truss did on Wednesday, Mr. Sunak was handicapped by the belief among members that he had betrayed Mr. Johnson. His resignation in July triggered the downfall of the prime minister, who despite a string of scandals remains enduringly popular among the party rank and file.
Rather than confront the economic headwinds, Ms. Truss and Mr. Sunak have waged a surprisingly bitter battle against each other. After one of their first head-to-head debates in July, aides to Ms. Truss accused Mr. Sunak of “mansplaining” and speaking over her. He said her policies would “tip millions of people into misery.”
More recently, Mr. Sunak has signaled he would not serve in Ms. Truss’s cabinet if she became prime minister. The rancor has left the Conservative Party deeply divided, with some members yearning for the return of Mr. Johnson, who has never explicitly ruled out a comeback in the future.
The new leader will travel to Scotland on Tuesday for an audience with Queen Elizabeth II at Balmoral Castle, where she vacations in the summer, and where she will also bid farewell to Mr. Johnson.
The ceremonial transfer of power is usually held at Buckingham Palace in London, but has been moved to Balmoral to accommodate the 96-year-old queen, who is in fragile health. Palace officials said they did not want to risk disrupting government business if the queen planned a return to London for the occasion and then suddenly had to cancel.
The winner will inherit one of the most challenging lists of problems to face a British prime minister in generations. In addition to the economy, Britain will face pressure to maintain its support of Ukraine and its sanctions against Russia. It may also be entering a new period of turbulence in relations with the European Union.
Mr. Johnson’s government angered Brussels by introducing legislation that would overturn the trading arrangements that govern Northern Ireland. Both candidates have vowed to push the legislation through Parliament, despite warnings that it could trigger a trade war with the European Union.
Ms. Truss asserted her credentials as a foreign minister, declaring that Britain should consider supplying heavier weapons to the Ukrainian army and impose further sanctions on President Vladimir V. Putin of Russia.
But she stayed away from inflammatory comments about foreign leaders. After saying last week that the “jury is out” on whether President Emmanuel Macron of France was a friend or foe, Ms. Truss dodged the same question about former President Donald J. Trump.
“I’m not going to comment on future potential presidential runners,” she said. “We have to work with whoever is in the White House.”