What should we make of Sarah Olney’s win in Richmond Park?

The Liberal Democrats’ stunning victory in the Richmond Park by-election confirms the transformative impact of the EU referendum on British politics. Sarah Olney overturned a 23,000-Conservative majority, defeating Zac Goldsmith, who had resigned his seat and stood as an independent. The result has exhilarated opponents of “hard Brexit,” but perhaps its greater effect is on the party system.

The major lesson of Richmond Park, which declared in the early hours of this morning, is that the Liberal Democrats are back from the brink. The party’s past success was built on presenting an anti-Conservative alternative in affluent areas where Labour was weak. The strategy combined tactical votes from left-leaning voters and protest votes from moderate Conservative supporters, and delivered major gains in 1997. However, the Lib Dems’ decision to join the Conservatives in a coalition in 2010 severely undermined their anti-Conservative image and led to a collapse in support in the 2015 general election. With just eight seats in parliament and lacking visibility in the media, the Lib Dems could easily have entered a death spiral.

Brexit may well end up saving the party by giving it a purpose and voters an incentive to vote for it. Theresa May’s eager pursuit of Brexit, combined with Labour’s weakness under Jeremy Corbyn, has created an opportunity for the Lib Dems to re-emerge. They are reclaiming the mantle of anti-Conservatism and reassembling the coalition of centrist Tory protest voters and Labour (and Green) tactical voters. The signs were there in the Witney by-election in October but Richmond Park has confirmed it. It was a classic Liberal Democrat campaign, seizing the opportunity of a high-profile vote in a prosperous area, which they flooded with activists.

But while the party will understandably herald its gain, caution is needed in drawing the lessons of the vote. Richmond Park is perfect Lib Dem territory—the party held the seat from 1997-2010—and as a London seat, it is in the only region of England that opted for “Remain” in June. Indeed, Richmond overwhelmingly voted 69-31 to “Remain.” The Lib Dems’ strength was previously concentrated in the South West of England, but that region voted “Leave.” If they are to return as a serious parliamentary force, they will need to win there. Circumspection is needed on the wider (un)popularity of Brexit. There has not been any noticeable shift in public opinion over Brexit since the referendum.

It is unclear how the circumstances of the by-election helped the party. The Conservatives did not field an official candidate, despite Goldsmith's resignation over Heathrow expansion. We don’t know whether a Conservative candidate would have performed better. Some Labour supporters appear to have been motivated to vote tactically to punish Goldsmith for what many saw as a disreputable and dirty campaign to become London mayor in May. It is an ignominious end to Goldsmith’s political career and opponents have already criticised the vanity of pledging to resign over a single issue rather than accepting that politics is a team game. Just because Goldsmith wanted to turn the by-election into a plebiscite on Heathrow did not guarantee that everyone else would agree. Indeed, he was undermined by the fact that all of the major candidates opposed expansion.

That left the campaign to become dominated by Brexit. Goldsmith was put at a distinct disadvantage by the fact that he was a Leaver in a heavily “Remain” constituency—and the Lib Dems exploited it mercilessly. Goldsmith lost despite UKIP choosing not to field a candidate in order to allow him to monopolise the pro-Brexit vote, but he still came up short.

The vote may give the government pause for thought in how it pursues Brexit, as it will be wary of fuelling a wider Lib Dem surge. It may also encourage Tory Remainers to argue the case for a softer Brexit. However, the government is still far ahead of Labour in the opinion polls and that will give it breathing space. A stronger Labour Party under credible leadership would have left the government looking much more vulnerable. But Richmond Park has shown that Leave-Remain is the new dividing line in British politics. On that basis, the next big by-election shock may well involve Labour and UKIP in a working-class Northern constituency.