As the Liberal Democrats mark their 30th birthday at their spring conference in Southport this weekend, there is not much to celebrate. Any prospect of the power they enjoyed during the Coalition is too distant to even dream about. They are not even the third party at Westminster. It hurts them to see several SNP MPs called at Prime Minister’s Questions every week. Vince Cable, the Lib Dem leader, gets one shot a month. Even some insiders admit the party has to answer the “what are we here for?” question.

Its unique selling point as the anti-Brexit party has not sparked the recovery it was banking on, as Jeremy Corbyn hoovers up the Remain vote. Cable reckons there is a 30 per cent chance of stopping Brexit, through Parliament voting for a referendum. But there is no guarantee the Lib Dems would then reap a dividend. They have slipped off the radar of too many voters.

Replacing Tim Farron with the more credible Cable has not improved the party’s ratings; it is stuck stubbornly at around 7 per cent in the polls. Some senior Lib Dems worry that Cable is repeating Farron’s mistake by putting too many eggs in the Brexit basket, and will have nothing to say after the UK leaves the EU in a year’s time.

Cable will answer that criticism by portraying his party as “fizzing with big ideas.” He will point to an earmarked tax rise for NHS and social care; replacing the Ofsted schools inspectorate and a graduate tax - what the Coalition should have called the hike in tuition fees which did the Lib Dems so much damage, and remains their (unwanted) USP in voters’ eyes.

The Lib Dems might look like they are sleeping but they are not dead. They are winning council by-elections again. They are cautiously optimistic about making gains at the May local elections and are pitching for the votes of EU nationals (though Labour is too).

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Cable is realistic about what his party can achieve. Gone are the days when the Lib Dems dismissed chatter about a new centre party by saying: “It already exists. Come and join us.” Today Cable wants his party to be at the heart of the “major shake-up” Brexit’s shockwaves might yet create, but doesn’t “want to be exclusive and tribal.” In other words, he wouldn’t expect pro-EU Labour and Tory MPs to join the Lib Dems if they put “country before party” and leave their tribe. A more likely scenario would be the Lib Dems joining a new centre force if one is forged (still a big if).

Cable’s consensual approach is reflected in a Lib Dem strategy document to be approved at the Southport conference. Much of it is motherhood and apple pie. It talks of building a loyal “core vote”, so the Lib Dems no longer rely on protest votes. But, significantly, the Lib Dems will commit to “working with those who share goals with us to build a progressive alliance of ideas that puts aside tribal differences to achieve shared goals.”

Cable is not talking about an electoral pact in which other parties stand down for the one most likely to defeat the Tories. This form of “progressive alliance” was tried hurriedly at last year’s snap election but looks unlikely to happen next time. The Green Party is sore that its decision to step aside in seats was not reciprocated by Labour and the Lib Dems.

However, there are early signs that such an alliance could re-emerge at the next election in a “below the radar” agreement under which the Lib Dems and Labour would not put energy and resources into the seats where the other party has a chance of beating the Tories.

Corbyn is a more tribal figure than Cable. His instinct would be to keep at arms length a party that hopped into bed with the Tories in 2010. And yet….Labour might need the votes of Lib Dem MPs, however small their number, in order to form a minority government after the next election. Corbyn’s project is no longer to merely transform Labour, but the country. “The leadership is itching to get their bums on ministerial seats at any price,” said one Labour insider. So why not run token campaigns in Lib Dem-Tory marginals that Labour couldn’t win anyway?