Understandably, there is much speculation over cracks in the coalition -- particularly on the Liberal Democrat side -- in the wake of the Budget and its small print, with cuts to incapacity benefit and unemployment to come.

But there is an even greater test for this government coming down the track: the referendum on the Alternative Vote, a subject I explore in the politics column of tomorrow's magazine.

At present, it is not in the news, not least because -- unlike the Lib Dems, who say it must happen next year -- the Tories have yet to confirm a date. But when it comes to it, whether or not the motion passes will be the real decider on whether or not there is a determined rebellion among Lib Dems at, say, next year's annual conference, after what will undoubtedly be a battering in the Scottish, Welsh and local elections for the third party next May.

The new Labour leader will have to decide whether or not cynically to oppose a "Yes" vote for AV in order to exploit coalition divisions for tactical reasons, or more authentically stick by support for AV as stated at the last election.

But an even greater dilemma faces David Cameron. In order to persuade his MPs to accept the Lib Dem demand for a referendum during the coalition negotiations, he made it clear they can campaign for a "No" vote. And there is no doubt he will stick to that. What is in doubt, however, is the assumption -- widely held across Westminster, including among Lib Dem cabinet ministers -- that he himself will back publicly the "No"' campaign.

Instead, in a sign of how deep the pact between himself and Nick Clegg may yet become, I understand that Cameron is considering supporting a "Yes" vote. He will not campaign for it like Clegg, but he could state his support much nearer the time.

Such a move would, incidentally, have the added tactical advantage of placing pressure on Labour not to be seen as conservative on electoral reform. More importantly, Cameron knows that -- unless he is in a strong enough position to win the election after a major Lib Dem revolt -- he must, though it will be against his instincts, do all he can to get a "Yes" vote.

David Cameron failed to produce a "Clause Four" moment during his five years as opposition leader. He then belatedly adapted to the new situation after the election by forcing his party to accept a referendum.

To back a "Yes" vote may infuriate some of his own backbenchers. Yet it would show the Lib Dems -- who have given much for little in return so far -- that he is serious about sharing power.

For details, see James Macintyre's column in the magazine out tomorrow.

UPDATE: There has been a query on attribution. Unfortunately you'll have to wait for the full piece to come out for that, but I can say that I have spoken to a number of Lib Dem and Tory parliamentarians -- both MPs and peers -- as well as strategists.