Cross-party efforts are being made to rebuild a grassroots alliance that could take Labour-Liberal Democrat co-operation out of the deep freeze before the next election.

The drive comes despite plummeting relations between the Liberal Democrat leadership and sections of Labour at Westminster, mainly over spending cuts and tuition fees.

Grassroots members of both parties are reaching out to each other, in the short term driven by the need for pro-constitutional reformers in both parties to work together ahead of the May referendum on the alternative vote due on 6 May.

But there is also a belief that the two parties will have to work together in the long term, and that centre-left Liberal Democrats can be strengthened against the small-state liberal faction.

Ben Bradshaw, a former Labour cabinet minister well disposed to the Liberal Democrats, is to front the Labour Yes to AV campaign. He was invited to take up the role late last week, and privately admits a huge amount of work needs to be done.

Labour is not expected formally to back the Yes campaign, because of internal divisions on the issue and the demands of some activists that the party should focus on local elections being held on the same day as the referendum.

Both Compass and Progress, the two leading left and centre-right factions inside the Labour party, will back the campaign and union funding is expected.

But supporters of AV inside Labour argue that a precondition of victory will be better relations between Labour and the Liberal Democrats.

The influential campaign group Compass is attempting to create a progressive alliance on the left, balloting its supporters on whether it should open its membership lists to members of other political parties including the Liberal Democrats and the Greens, as well as non-aligned.

Compass chair Neal Lawson, said: "We need to think how we can create a progressive consensus in this country, and one motive is to make sure that if there is another hung parliament a progressive force exists outside parliament that creates unstoppable pressures for a Labour and Liberal Democrat alliance." Compass will continue to operate independently inside the Labour party.

Will Straw, editor of the blog Left Foot Forward, has also engaged in the mapping of "centre-left forces" – called the Latimer project – to bring together Labour activists with Liberal Democrats and Greens but also to identify areas the left must rebuild if it is to have a chance of regaining power.

He has sought to learn lessons from two recent experiences of a party losing power only to re-enter after a significant period of renewal – the experience of the Democrats in the States and the Tories here in the UK, saying: "Both show that they wasted four years, and that there were four stages."

The first lesson was to change the debate through thinktanks. The second was that organisations like Migration Watch and the Taxpayers' Alliance helped the Tories move the centre ground.

The third was training up activist bases such as the website Conservative Home and the Young Britons Foundation. The fourth was that when the election was called they "unleashed the beast", in Straw's words, deploying groups such as the Countryside Alliance, which sent 20 activists to fight the general election in Calder Valley, distributing Tory party literature rather than its own. All of these factors in the UK, and their equivalents in the US, contributed to helping a party return to power.

In an attempt to look more closely at the second of these, in December there will be a series of briefings by senior figures about individual "outlier" organisations.

Labour backbencher Jon Cruddas will do the English Defence League; Clifford Singer will do a presentation on the Taxpayers' Alliance; Philippe Legrain on Migration Watch; Obama aide Karin Robinson dissecting the Tea Party and Tom Compton from the World Wildlife Fund on how to build "shared values" to combat climate change, "deep framing" issues.

Straw believes this will necessarily involve joining forces with Lib Dems, and yesterday's Latimer project meeting included Lib Dems. Straw said: "The ethos of the Latimer project is to work with all progressive forces and that includes people in the Lib Dems.

"We should be reaching out to those who are concerned with their party's direction of travel who are very different from those in the Kool Aid-drinking Lib Dem leadership. Labour may well be forced to work with the Lib Dems on some of these things … It is a mistake for those in the Labour party to try and portray all the Lib Dems as traitors."

There are tentative signs that left-leaning Liberal Democrat grassroots figures such as Evan Harris are in the ascendancy in the party, with Harris easily topping the poll for the party's federal executive, which steers party strategy, suggesting those that favour links with Labour remain strong in the party.

Nick Pearce, a former senior adviser to Gordon Brown and deeply critical of the Liberal Democrat role in the post-election talks, has also urged Labour to rethink its approach: "Labour was too tribal for too long. It didn't start preparing for coalition talks until very late in the day, long after the other parties had given it a great deal of thought and preparation.

"Coalition government is likely to become the norm in Britain, even if the AV referendum next year is lost. That means that political leaders need to get used to negotiating in good faith about forming governments. In particular, if Labour and the Liberal Democrats are ever to form a coalition government in the future, the bitterness that has marked their relations since those five days in May will have to subside."

• This article was amended on 17 November 2010. The quotation: "It is a mistake for those in the Labour party to try and portray the Lib Dems as traitors" was corrected to: "It is a mistake for those in the Labour party to try and portray all the Lib Dems as traitors."