Jeremy Corbyn has always been more of a politician than either his most fervent supporters or detractors have wanted to admit. Last night he again demonstrated why. 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.

There might be debate about why he’s done this, but ultimately his motives don’t matter. Corbyn is the leader of the opposition and, in accordance with our unwritten constitution, the first alternative prime minister. He is also offering a concrete proposal to do exactly the thing remainers say they want – to stop no deal and then offer voters the chance to stop Brexit altogether.

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. Instead, she has declared she could support a Ken Clarke or Harriet Harman government. All well and good, but neither of these MPs is leader of the opposition (or any political party), and Swinson does not have the parliamentary numbers or time to pick and choose who she is prepared to work with.

The new Lib Dem leader risks making a grave mistake. Even in purely party political terms, a Corbyn-led caretaker government does not necessarily strengthen Labour in the long term. But more importantly, Swinson has always emphasised, rightly, that her party’s priority is to stop no deal. This could prove the only way to do so. If the Lib Dems really believe that a few months of a limited Corbyn government is worse than medicine shortages, it is their duty to say why.

Play Video 1:41 Jo Swinson says Corbyn cannot become temporary PM – video

Remainers can, of course, be forgiven for some scepticism at Corbyn’s offer, and many questions remain unanswered. Labour will offer a remain option in a referendum, but will it in all circumstances campaign for that? Is it still party policy to renegotiate Brexit first? Why not in fact call that referendum now, then hold a general election that – heaven forbid – might not actually be about Brexit, but instead focus on some of the vital issues we have been ignoring for the past three years? The point is that all parties will need to engage, fast and in good faith. Any prime minister derives their authority from parliament alone, and if Corbyn strayed from the agreed path MPs could instantly withdraw their confidence.

It is also the case that parliament can deploy multiple and simultaneous tactics to stop no deal. MPs can enact legislation to force an extension request while still haggling over no-confidence motions and an alternative government. It is, however, more difficult to legislate in the face of government opposition than to work with a government that shares your basic objectives. Corbyn’s solution may not be perfect, but may be the most viable.

Corbyn has taken a brave step. Now he must rule out any ‘Labour Brexit’ | Paul Mason Read more

The truth is that if our political parties do not do everything possible to avoid no deal, none of them will escape the blame for it. When politicians put party or personality ahead of the national interest and people’s lives, voters punish them. Such a spectacular failure of the political class would only benefit the extreme political fringes.

It is time for the Lib Dems, and indeed all remainers, to decide what they really want. A few months of a Corbyn government is not worse than infrastructural or economic collapse. If your most important goal is to stop no deal, you must take every conceivable step to do just that. Our politicians will not benefit from the catastrophe of a crash-out Brexit. But, far more importantly, neither will the British people.

• Jonathan Lis is deputy director of the thinktank British Influence