In a London special election, a popular candidate aligned with the Conservatives lost to an opponent of leaving the European Union, raising issues about how deep Britain’s departure will be.
LONDON — An election just contested here was supposed to be a referendum on the expansion of Heathrow Airport, and only of local interest. But it turned into something far more profound: a referendum on the referendum for Britain to quit the European Union, and its results sent tremors through the Conservative government of Prime Minister Theresa May and made the Labour Party look irrelevant.
The race for a seat in Parliament was won on Thursday by the candidate of the diminished Liberal Democratic Party, which is calling for another referendum on the terms of Britain’s departure from Europe, known as “Brexit.” Its candidate, Sarah Olney, defeated Zac Goldsmith, who had held the seat representing the London borough of Richmond for the Conservatives since 2010.
It was not just that Ms. Olney won, but how. She defeated Mr. Goldsmith by about 2,000 votes out of 41,367 votes cast, after Mr. Goldsmith had a 23,000-vote margin in the previous election.
The Labour candidate drew so few votes, around 1,500, that the party was forced to forfeit a deposit of 500 pounds ($634) that is extracted from nuisance candidates and typically refunded to major parties.
The trouble for Mr. Goldsmith — and, by extension, the broader Conservative Party — was that he had been a vocal advocate for Britain leaving the European Union. But in the June referendum on whether to leave the bloc, his constituents voted to remain by about 70 percent to 30 percent.
Thursday’s contest was a by-election, a special vote used to fill vacant seats between general elections. They often turn into tools for protest voting, and this was no exception. The vote showed the deep and growing anger and anxiety within Britain about how the country should manage its exit from the bloc, under what terms and with what future relationships.
People who voted in the June referendum to remain are urging a close relationship with Europe, including access to the single market. The Remain voters say that they have the right to speak out even if they lost, and that the referendum was a strict in or out vote, without details about the future.
But those on the Leave side are angry and defensive, too. They fear that their victory will be somehow snatched away by elites, and that even Mrs. May, who favored remaining, will not follow through on leaving.
This vote also cuts her narrow majority in the House of Commons even more, raising new speculation about whether she will seek an early election to win her own mandate to lead.
The by-election became necessary after Mr. Goldsmith, who is popular in Richmond, had promised to quit if the Conservative government decided to expand Heathrow Airport with a third runway, which it did. So he resigned, hoping to pressure the government to rethink.
He decided to run again for his old seat as an independent, and the Conservatives did not even put up an opponent. But without the party’s support, he had no access to the party’s voter database.
Ms. Olney also opposed the new runway at Heathrow, as did the Labour candidate, Christian Wolmar, making moot Mr. Goldsmith’s pet issue. Instead, Ms. Olney urged voters to send a message to the government that the 48 percent of Britons who voted to Remain were not to be ignored.
“Our message is clear: We do not want a hard Brexit,” she said. “We do not want to be pulled out of the single market, and we will not let intolerance, division and fear win.”
Many who favor leaving want to get out of the alliance at all costs, a so-called hard exit, to restore complete British control over immigration and the budget. But others, like Ms. Olney, want to maintain duty-free access to the bloc, even if that means continuing to pay into the alliance’s budget or to continue to accept the free movement of labor.
The main opposition Labour Party has been ambivalent about the issue and its candidate, Mr. Wolmar, did poorly in the wealthy Richmond area. Worse, it made Labour look irrelevant in the main issue of the race.
The Liberal Democrats, by contrast, poured enormous resources into Richmond. Stung in the 2015 national elections and reduced to only eight members in Parliament, the party’s new leader, Tim Farron, saw Richmond as a chance to resurrect support.
“This was not just about a Remain versus Leave rerun,” Mr. Farron told the British Broadcasting Corporation on Friday. “This was about people trying to say to Theresa May, ‘We do not like the extreme version of Brexit outside the single market you’re taking us down.’”
The Liberal Democrats invested in an enormous get-out-the-vote operation. They had 600 activists on the ground, with another 241 people staffing a phone bank that made 13,000 calls, and knocked on 36,000 doors, according to the British news media.
The result in Richmond could signal a new beginning for the Liberal Democrats, who traditionally do well in voting for local councils. But as a columnist for The Guardian, Rafael Behr, suggested in a post on Twitter that the victory for the Liberals was like a “cup of tea in emergency shelter after flood has washed the house away. Nice, but not a new house.”
However heartened some who voted to remain are by the Richmond result, opinion polls suggest that national attitudes have not shifted much since the June vote, and have actually hardened.
But the by-election’s result unmistakably underlines the political difficulties facing Mrs. May, who has yet to develop a clear negotiating strategy for Britain’s withdrawal from Europe.
So far, she has played for time. She said that she would begin the process of withdrawal from the bloc by the end of March. By then she and the government will have to make some decisions about priorities and about what the departure means: whether to remain within the single market or the customs union, and if so, at what price? Or does control over immigration take precedence over every other consideration?
One of her difficulties is that, if she hints at a pragmatic approach to leaving, she risks upsetting the right wing of her Conservative party.
The result in Richmond also underlines the volatility of the electorate, which may reinforce Mrs. May’s natural caution, making an early election less likely for her.