According to polls, the centrist Liberal Democrats party (Lib Dems) are a critical obstacle to Labour taking over the UK government.
With the deeply unpopular Torie PM Boris Johnson leading the charge for the deeply unpopular No-Deal Brexit, Jeremy Corbyn made an extremely unusual proposal to stop it from happening.

In a move unprecedented in modern times, the leader of the opposition has offered to form a government with the express proviso of not implementing any of his party’s policies.

Corbyn’s offer, in a letter to other party leaders and moderate Tories, involves setting up a “strictly time-limited temporary government” with the sole intention of extending article 50 and holding a general election. In that election, Corbyn will commit to a new referendum with the option of remaining in the EU.

The Tories only have a one-seat majority, so this was a realistic proposal.
The SNP, Greens, and Plaid Cymru all agreed, and several Tory backbenchers planned to discuss it.
All that was needed was for the Lib Dems to sign off on the plan.

In the middle ground between the dominant Labour Party and the Conservative Party, the Liberal Democrats occupy a centre-left, libertarian position.
- ENCYCLOPÆDIA BRITANNICA

Since the Lib Dems got killed in the 2015 election they've been a one-issue party - overturn Brexit.

Of course, if Corbyn was attempting to trap the Lib Dems, they have walked right into it. Jo Swinson, the party’s leader, has dismissed outright the prospect of Corbyn leading such a government, and has not even signalled a willingness to enter discussions with him.

Wow. What happened here?
What happened was about social class. Think of the Lib Dems as Clintonites. The Lib Dems represent upper-class globalists.

If your signature policy – indeed, only visible political position – is to stop Brexit, and you claim that you will do absolutely everything within your power to prevent no deal, then it’s something of an error to suddenly introduce an exception. And yet this is the fatal mistake the Liberal Democrats have made...
Instead, Swinson revealed that while the Lib Dems had been willing to prop David Cameron up for five years, implementing massive cuts and trebling tuition fees, she’s not prepared to countenance supporting Corbyn for five weeks solely to stop a disorderly exit from the EU.

The class bias of the Lib Dems is so deep that they would rather betray their identifying issue - their reason to exist - than work with Labour for five weeks. This is what they were saying about the Lib Dems just a month ago.

The party is now gaining support from disillusioned Labour voters who are alarmed at Corbyn’s socialism and frustrated with his ambiguous position on Brexit...Liberal Democrat success could be the Tories’ best hope of winning an election.

They just blew that up.
All of a sudden Labour just became the strongest party of hope for Remainers (i.e. a majority of the UK), the Lib Dems exposed themselves for what they are, and Corbyn did all this without having to commit to Remain. He only had to commit to a referendum.
It was a brilliant political coup.
The Lib Dems just kneecapped themselves. Watch as their supporters become Labour voters.