LONDON — Prime Minister Boris Johnson went to the mat yet again on Wednesday for his embattled adviser, Dominic Cummings, signaling to lawmakers that he had no plans to dismiss Mr. Cummings for breaching the rules by driving 260 miles to his parents’ house during the country’s coronavirus lockdown.
But Mr. Johnson deflected the most telling question in a closely watched and often-contentious hearing: Why was he clinging to an aide who is so obviously damaged goods?
“You have a choice between protecting Dominic Cummings and putting the national interest first,” said Yvette Cooper, a Labour Party lawmaker, in a particularly heated exchange. “Which will it be, prime minister?”
“My choice is the choice of the British people,” a flustered Mr. Johnson replied, before accusing Ms. Cooper of trying to score political points when Britain needed to move on from a dispute that has inflamed public opinion, divided his Conservative Party, and eroded the prime minister’s popularity.
“What they want now is for us to focus on them and their needs rather than on a political ding-dong about what one adviser may or may not have done,” Mr. Johnson told a parliamentary committee.
Eager to change the subject, he announced plans for a large-scale track and trace system to head off a second spike in infections. Anyone suffering symptoms will be tested and, if positive, be asked to list all those with whom they have recently been in close contact for at least 15 minutes. Those people, in turn, will be contacted and asked to isolate themselves for 14 days.
But the fallout from the Cummings affair overshadowed that news, as lawmaker after lawmaker challenged Mr. Johnson about how the government planned to force people to disclose their contacts or go into quarantine.
Scientists have warned that Mr. Cummings’s breach of the lockdown rules will undermine public compliance with that system, as well as with the new quarantine regime for arriving travelers that begins next month.
Ms. Cooper pressed Mr. Johnson on whether parents with extraordinary child care needs, such as what Mr. Cummings claimed in his case, could ignore the government’s guidance to stay home, even if they had symptoms of the virus, as Mr. Cummings’s wife did. The prime minister said it depended on the circumstances.
Simon Hoare, a Conservative lawmaker, asked Mr. Johnson whether he understood the depth of anger at Mr. Cummings’s behavior, which sent a signal that senior officials did not have to abide by the rules that applied to everybody else.
“I do understand people’s feelings,” Mr. Johnson said. “I do understand why people feel such indignation.”
The prime minister’s halting testimony laid bare, once again, how reliant he has become on Mr. Cummings. The 48-year-old strategist has shaped the agenda of the Johnson government, an ambitious plan known as “leveling up,” which aims to close the economic divide between London and northern England by spending on high-speed rail networks and other big-ticket projects.
Mr. Johnson, critics note, has always been more focused on amassing political power than on exercising it for particular purposes. He has little fixed ideology and is capable of shifting positions for political expediency. Mr. Cummings, on the other hand, brims with ideas, not all of them practical. He, too, resists ideological labels but is fueled by an abiding suspicion for established institutions.
Since moving into Downing Street with Mr. Johnson last July, Mr. Cummings has seeded government ministries with loyalists and centralized economic policy in the prime minister’s office. His aggressive tactics contributed to driving out Mr. Johnson’s first chancellor of the Exchequer, Sajid Javid, who was replaced with a younger, more compliant substitute, Rishi Sunak.
Mr. Johnson’s loyalty to Mr. Cummings has already exacted a high political price, in terms of his personal popularity and the credibility of his government. The longer Mr. Johnson sticks by his aide, critics said, the worse the damage will be if he ultimately decides he has to cut him loose.
“He’s got a very binary choice,” said Jonathan Powell, a former chief of staff to Prime Minister Tony Blair. “Either he looks weak by letting the guy go, or he hangs on and it affects the government because the guy is toxic.”
In two decades in politics, Mr. Cummings acquired a fearsome reputation as a campaigner and disrupter, culminating in his pivotal role in the 2016 Brexit referendum that persuaded Britons to quit the European Union.
But he has few allies in the Conservative Party, an organization he appears never to have joined. He has been as likely to disparage Conservative backbench lawmakers as to cultivate them — one reason, perhaps, that about 40 of them have called on Mr. Cummings to resign or be fired.
Ideological fellow travelers have not been spared either. He once said that members of a hard-line pro-Brexit caucus in Parliament, the European Research Group, “should be treated like a metastasizing tumor and excised from the U.K. body politic.”
During the referendum, he observed that some of these Conservative pro-Brexit lawmakers were too busy “shooting, skiing or chasing girls to do any actual work.” Among those he singled out for criticism was Bernard Jenkin, the chairman of the parliamentary committee that met digitally on Wednesday.
Others who felt the lash of Mr. Cummings’s tongue include the former party leader, Iain Duncan Smith (“incompetent”), and the former Brexit secretary, David Davis (“thick as mince, lazy as a toad, and vain as Narcissus”).
Now, Mr. Cummings’s lack of friends in the party has come back to haunt him. Several Conservative lawmakers who called on him to resign pointedly mentioned, in doing so, that they had never met him. They included Douglas Ross, who resigned as the under secretary of state for Scotland in protest; Mark Harper, a former chief whip; and Harriett Baldwin, a former minister.
Others simply reacted badly to what they saw and heard. “It’s his cavalier, ‘I don’t care; I’m cleverer than you’ tone that infuriates people,” Mr. Hoare, the chairman of the House of Commons Northern Ireland Affairs select committee, wrote on Twitter before the committee hearing in which he took part. “I don’t like that,” he added.