One reason why the Lib Dems are less despondent than their poll ratings would suggest is that the party could still hold the balance of power after the next election. The Conservatives will struggle to win the majority they failed to secure in 2010 - no governing party has increased its share of the vote since 1974 - but the party's existing support could prove sufficiently resilient to prevent Labour getting over the winning line of 326 seats. For this reason, the Lib Dems, who expect many of their sitting MPs to defy the national swing, believe that Ed Miliband's party could be forced to turn to them for assistance in another hung parliament.
There are now signs that Labour is acknowledging as much. Yesterday on The Staggers, former cabinet minister John Denham, previewing the launch of a new group called Labour for Democracy , wrote that  "rather than Labour re-establishing itself as the sole party of choice for progressive voters, it's more likely that the progressive vote will be split as it has now been for decades." In other words, the Lib Dem vote will prove more durable than many in Labour currently assume. It is a message reinforced by Labour for Democracy, which states on its website: "All Labour members will work hard for every Labour vote. But whether we win the outright Labour majority we all seek, or end up with a less conclusive result, the change Britain needs will require the support of all who share our values."
Though the words "tactical voting" do not appear in Denham's piece, his prediction that the progressive vote "will be split" is a reminder that it could play an important role at the next election. The Conservatives are in second place in 38 of the Lib Dems' 57 seats and half of those  on its target list are held by Clegg's party. If Labour is to prevent the Tories from decapitating scores of Lib Dems, it will need to consider whether to advise its supporters to cast tactical votes (as Peter Hain and Ed Balls came close to doing at the last election ).
Denham goes on to note that "despite the failures of the coalition, the public still generally want politicians to work together when they can, rather than exaggerate their differences." What makes his intervention particularly significant is that he is now PPS to Miliband (he was one of the Labour leader's early shadow cabinet supporters), suggesting that he is acting with the approval of the leadership. He concluded:
It is tempting to see pluralism as a sign of weakness, a lack of confidence; even an unwanted attempt to give Nick Clegg a permanent and undeserved place in government.
But we must be bigger than that. Tribal differences have obstructed progressive change in the past. Voter allegiances to the major parties are declining as fast as the icecaps are melting. There are even signs that the ‘progressive majority’ that split its vote in the 1980s is itself shrinking in the face of recession and insecurity. If we want to change Britain in a progressive direction, Labour must show it is willing to work with, not just lead, everyone who will support all or part of that change.
So, while Clegg's head remains the price  of a Labour-Lib Dem coalition (indeed, Denham was one of the first  senior Labour figures to state as much), Labour is increasingly clear that it may not be possible for it govern alone after the next election. Expect Denham's article to mark the beginning of a prolonged rapprochement.