Boris Johnson was elected leader of Britain’s Conservative Party last Tuesday, became prime minister on Wednesday, then set about purging high-level leaders in his own party (one tabloid’s headline Thursday morning: “Night of the Blond Knives”). But Johnson’s government has only a three-seat majority in parliament, and his willingness to leave the European Union without an exit deal in place—a so-called no-deal Brexit—has put him at odds with many senior figures in his own party. Meanwhile, Brexit is less popular than ever: Surveys now regularly show that a majority of voters would rather stay in the EU, sometimes by double-digit margins.

That’s why Johnson’s election as Conservative head might not have even been the United Kingdom’s most consequential party leadership contest last week. That Monday, the staunchly pro-European Liberal Democrats elected Jo Swinson as their new leader. Swinson, at 39 the youngest major-party leader by over a decade, pledged to do “whatever it takes to stop Brexit.” Despite leading just the fourth-largest party in parliament, Swinson is remarkably well positioned to capitalize on backlash to Brexit and the Johnson-led government, as well as allegations of widespread institutional anti-Semitism within the leftist Labour Party. When she claimed to be not just the leader of her party but, in fact, “a candidate for prime minister,” it was not the laughable statement it might have been just a few months earlier.

The Liberal Democrats had been consigned to the electoral ossuary as recently as last winter, limping along at around 8 percent in most opinion polls, and discussing bringing in a celebrity leader to raise the party’s profile. Then, erstwhile Prime Minister Theresa May’s Brexit negotiations resulted in an unpopular deal that she was unable to push through parliament on three separate attempts, once losing by a stunning 230 votes. The right wing of May’s party, the Conservatives, began pledging to take the country out of the EU without a departure deal, an action economists think could prove catastrophic for many in the country (including industries such as agriculture and finance, which rely on trade with Europe and employ millions).

Almost overnight, everything changed for the Lib Dems. They scored a massive success in the country’s local elections in May, then, later that month, won the second-highest number of votes and seats in the European Parliament elections, pushing Labour into third place and the Conservatives into fifth. Now, recent polling shows the Lib Dems in second place nationally, close behind the Conservatives. One poll, taken just after both Johnson’s and Swinson’s elections, found their parties essentially tied, with Labour struggling in third.

Still, the Liberal Democrats have just twelve representatives in the 650-seat House of Commons. Johnson’s Conservative Party has 311, and Labour has 247. But it’s the Lib Dems that are in the best position to capitalize on the turmoil of Brexit. The party has promised to keep Britain in the EU. Johnson would take the country out of the bloc without a withdrawal agreement if he cannot renegotiate the exit deal (EU representatives have said the agreement is not up for renegotiation) and the Labour Party has taken a position so milquetoast and vague that not even veteran political journalists pretend to know where the party stands.