I don't really know if the Tories "Vote Clegg, get Brown" argument will work but if I had to bet on it I'd guess that it won't. There's a large enough constituency out there that doesn't want either the Tories or Labour.
Nevertheless, the post-election environment now becomes very interesting. Suppose, just for now, that Labour come third in the popular vote but actually win the most seats. What happens then? And this could happen. Consider this scenario: Tories 33%, Labour 28%, Lib Dems 29% - according to Electoral Calculus and, admittedly, on a crude national and uniform swing, this could produce a result something like Tories 258, Labour 265, Liberals 95.
Now clearly this wouldn't be any kind of actual vote for electoral reform but it would produce a post-election environment in which there was considerable enthusiasm for and momentum behind voting reform.
This being so one could imagine a situation in which a Labour minority government soldiered on for a year or so with tacit support from the Liberal Democrats until a new election was called and held on the same day as a referendum on some kind of PR system. I suppose the Lib Dems could just about manage to pretend that this one kind of "change" but I'm not sure that can hold.
How can you really ask people to vote for change and then end up by supporting the mob you've asked people to vote against? What kind of change is that?
This being so, a Tory-Liberal coalition makes much more sense, not least in as much as it would actually deliver a proper change of government which, in as much as anything is clear at the moment, would seem to be the one thing that more than 70% of the electorate can agree on.
What the rising Liberal tide has done, however, is raise the prospect of quite widespread tactical voting on a level we've not seen since 1997. It seems to me, then, that the opposition's first job has to be to ensure that Labour aren't the largest party: in some places that means voting Tory, in a few others it means endorsing the Liberals.
This is imperfect for the Conservatives of course since not everyone will remember to cast their ballot in the most efficient fashion and the Tories will lose seats they might otherwise have expected to gain. Nevertheless, we are where we are.
And it's not as if there isn't plenty of common ground for the Tories and Liberals to work together on. The "Big Society", civil liberties, decentralisation, localism, public spending restraint and so on provide plenty of room for the parties to work together, whether formally or in an informal arrangement.
Yes of course there'd be problems and important differences would remain but many of these could be worked out or elided or simply ignored for convenience's sake. Even electoral reform might not be a bridge too far: there's the safety valve of a referendum and, given that the Conservatives have made much of letting the people's voices be heard in other referendums it's hard to see how the party can have any genuinely principled opposition to just asking the question.
Still, the question for now might better be asked of Clegg: how can you really be the change candidate if you end up propping up Labour in any way? That, I'd have thought, is the one thing people really aren't voting for. This doesn't mean there's rampant enthusiasm for a Conservative-Liberal arrangement but that's a different matter entirely...