AN EXPECTED 100,000 people will march on Parliament Square in London on June 23rd to demand a “people’s vote” on Britain’s relationship with the European Union. The march will occur two years to the day since 33.5m people voted in a referendum to decide whether Britain should leave the EU. Contrary to the expectations of pollsters, pundits and punters, the Leave camp won by 1.3m votes.

Since then, Britain’s government has been mired in the sticky business of deciding what Leave actually means. Formally, the country will depart at 11pm on March 29th, 2019. It will probably then enter into a 21-month-long “transitional” arrangement, in which little will change substantively. Brexit proper will be due to happen on January 1st, 2021—fully four and a half years after the referendum.

By that time, it is likely that enthusiasm for leaving the world’s second largest economy will be further diminished. Ever since June 2016, YouGov, a pollster, has asked a representative sample of Britons at regular intervals how they might vote in another referendum on EU membership. Support for leaving has declined notably. For the past six months, all of YouGov’s polls have shown a clear lead for Remain over Leave. Their latest survey reported that 45% of respondents wanted to remain in the EU, 40% wished to leave, and the remaining 15% were undecided.

That sentiment has turned may not be surprising given how Britain voted. Broadly speaking, the old wanted to leave while the young wished to remain. In a recent article for Prospect magazine Peter Kellner, a former pollster for YouGov, calculates that since June 2016 deaths of the (mostly) old have reduced the ranks of Leave voters by 600,000, and those of Remain voters by 300,000. During the same period, an additional 650,000 would-be Remain voters have become eligible to vote, compared with just 150,000 additional Leave voters. Together, the net effect of demographic change alone—holding sentiment within each age group constant—is to reduce Leave’s lead by 800,000 votes. By the end of 2018, Mr Kellner estimates, demography, assuming voting patterns by age remain as they were in June 2016, will mean “Remain” would win a second vote.