LONDON — Politicians campaigning for and against British withdrawal from the European Union fanned out on Wednesday in a final, frenetic effort to build support on the eve of a referendum that could reshape the nation’s place in the world and upend the Continent’s dreams of closer integration.
With polls showing a statistical dead heat, both sides went all-out to motivate their supporters to vote on Thursday, while financial markets and European leaders braced for the possibility that Britain could be the first nation to leave the 28-member bloc. Reflecting the stakes and the tension about the outcome, the tone of the campaigning remained negative to the end, complete with invocations of economic ruin and an allusion to the Nazis.
Voters will be asked a single, simple question in the referendum: “Should the United Kingdom remain a member of the European Union or leave the European Union?”
A record number of voters — just shy of 46.5 million — have registered to take part. Polls will be open from 7 a.m. to 10 p.m. British time. The country’s Electoral Commission said it anticipated a result on Friday morning “around breakfast time,” but cautioned that there was “considerable uncertainty” around the timing.
The issue of a British exit, or Brexit, has deeply divided the governing Conservative Party, split families and neighborhoods, underscored the shortcomings of the Brussels bureaucracy, and fueled the hopes of anti-EU populists across the Continent.
Prime Minister David Cameron closed out the campaign for remaining in the European Union with an argument that Britain will be more prosperous if it stays in the single European market of 500 million people — and he warned that there was no going back from a decision to leave.
“You can’t jump out the airplane and then clamber back through the cockpit hatch,” he told the BBC.
In a last-minute controversy, Michael Gove, the justice secretary and a leader of the campaign to leave the bloc, likened economists who warned of the dire consequences of withdrawal to Nazi-financed researchers who once denounced Einstein. (Cameron said that Gove had “lost it,” and Gove apologized on Wednesday.)
Some polls showed the Remain side with a slight edge and others showed the Leave side ahead, but in most cases surveys showed the race within the margin of sampling error. A “poll of polls” compiled by The Financial Times found that the campaign was ending close to a dead heat.
Financial markets have been more confident that Britain will vote to stay in the bloc, with the pound rising in value against the dollar this week. On Wednesday, the pound held steady, close to its high against the dollar this year.
Bettors, too, have been putting their money on the Remain camp’s winning. On Wednesday afternoon, the odds implied an 80 percent chance of Britain voting to stay with Europe.
One sign of hope for the Remain campaign: Most polls before the September 2014 referendum on whether Scotland should secede from the United Kingdom showed that contest to be neck-and-neck, but voters broke fairly decisively — 55.3 percent to 44.7 percent — in favor of the status quo. Some analysts say the same tendency of late-deciding voters to break toward the safety of the status quo and away from the risk of change will be at play this time around.
Scotland is also strong territory for the Remain camp, though there is some evidence of political fatigue there.
“My worry is turnout in Scotland and UK-wide,” said Alyn Smith, a member of the European Parliament for the Scottish National Party, who backs the Remain camp. “It is quite clear that the people who have committed themselves to leave will crawl across glass to get to the polling stations and on the Remain side it’s much more, ‘Why are we doing this?’”
Matthew Goodwin, a professor of politics and international relations at the University of Kent, has written that the Remain camp was apparently more strategic in focusing its efforts on areas of strong support, while the Leave campaign had not done enough to lure votes in working-class areas of northern England where its message has resonated.