There seems to be some to-ing and fro-ing about how the Lib Dems should be treated by the opposition. There’s a simple answer to this: they should be treated as if they no longer exist. This is, after all, how Nick Clegg is treating them, and he should know.
People think of the Lib Dems as an institutional party in the same sense as Labour and the Tories. That was maybe true when the Liberals were one of the big two. Since then, they’re probably better regarded as a series of projects designed to get a share of power for their senior managers. As such, they’re quite a fissile bunch. Over the last century, you’ve had Liberal Unionists, Coupon Liberals, National Liberals, and now Liberal Democrats. This isn't to say that individuals who cluster around whatever formation is currently trading under the liberal marque don't have convictions and loyalties in the same way Labour and Tory members do. It's just that if your convictions and loyalties don't build up over the years to much more than a bargaining position after a tight election, you have to decide whether to soldier on or cash out. And as it happens, when the opportunity comes to cash out, the leadership of the various Liberal projects has always broken to the Tories, whatever the general sentiment among the people they recruited might be.
I think that was more a question of the way the numbers broke, rather than a matter of conviction. Back in the eighties, the Liberals, as was, made quite an audacious attempt to replace Labour as the main party of the left in Britain in conjunction with some Labour defectors, thus kicking off the whole process which eventually got them in government as a stick-on Tory left. And if if the Tories had got thirty fewer seats at the last election and Labour thirty more, we’d be seeing Nick and Vince saying that they’d taken a look at the real books and were horrified by all the warrantless Tory scaremongering about the size of the deficit, and yay the progressive coalition.
Anyway. The third party project named Lib Dem is now over. Its mission is fulfilled. That’s not to say that the Lib Dem apparatus will now simply dissolve. There has to be a period of cooling out the marks, as the saying goes, and for the Tories to embed as many as they can of the voters Clegg and Co have delivered to them. That process can last quite a long time. I don’t think the National Liberals formally vanished until the 1970’s – hell, I think Michael Heseltine was first elected as a National Liberal. At the very least, the Lib Dems will go forward as a hollow institution until we get some evidence of which of their voters simply revert to Labour, which of them find their current political environment quite comfortable and decide to vote for the organ grinder rather than the monkey, and which make a break for it and start all over again.
As for how Labour should respond, it could do a lot worse than adopt some of the policies that led a lot of left leaning people to find the Lib Dems attractive in the first place. All that’s left of them of interest to anyone else is a pool of voters, and it makes sense for the opposition to grab its share. But treating the Lib Dems themselves as a formation with content and meaning is irrelevant and potentially distracting to the job of opposing the government.