But Nick Clegg would have played his own hand rather badly if he could not secure a much more substantive move on electoral reform than David Cameron's offer of the well-worn gambit of a Royal Commission.
If Cameron could not table something more credible, his main tools to insist on a deal could be to threaten the LibDems with the fury of the Murdoch/Dacre press and the British electorate. Yet one lesson of last week is that the Tory instinct about these things is far from infallible. (And the second election is a risky undertaking: twice in the last century, the British electorate was asked to vote twice in the same year. Both times they returned almost exactly the same verdict the second time around. If, as a minority PM Cameron sought a majority mandate and failed, who could then question the legitimacy of a progressive combination to unseat him?)
The always well informed Ben Brogan says that the Tory line remains no compromise on first past the post.
Yet Cameron could easily go much further on electoral reform than most observers have suggested, in a wide range of ways, without provoking his party to mutiny further.
To start, various important sounding side-offers could be offered very easily and at no cost to any Tory agenda. A quick move to begin legislating for a PR-elected House of Lords could look a tempting bauble to LibDems. (An elected Lords is formally Tory policy, even if they didn't plan to do much about it, while majoritarian arguments do not apply to a revising chamber which does not sustain an administration).
Cameron has already proposed fixed election dates himself, so it would be rather strange to refuse that request, without which stable coalitions or parliamentary deals risk being a suicide pact for the smaller party.
Even proposing PR for English local government would be much easier than it looks. That could look like a significant Tory sacrifice when they have so many Councillors. In reality it would cushion the all but certain sharp decline from their current peak if they do govern nationally. And "no overall control" is already the norm in many councils. When Labour has delivered PR for Scotland, Wales, London and the European elections, the LibDems should be able to get some substantial progress of this kind if their Tory would-be allies want to claim to be serious about political reform.
What the LibDems really want is a referendum on PR for the Commons "Let the People Decide". So, what, really, is to stop Cameron conceding that a LibDem-Tory alliance would hold a referendum on an 'agree to differ' basis (such as that which Wilson used over Europe within a Labour cabinet in 1975)?
The pro-PR forces would face an uphill battle. Public opinion is for PR, but it is shallow and contradictory. There are majorities for PR as the only way to fairly distribute seats and, in the very same polls, for the current system to deliver single-party majority governments. So a (decisive) quarter of the electorate is for and against PR at the same time. This JRRT state of the nation poll in summer 2009 was typical:
This country should adopt a new voting system that would give parties seats in Parliament in proportion to their share of the votes.
63% agree with that, and only 22% disagree.
But here is the next but one question, to the same 1000 people in the same poll.
The present system of voting is the only way that the country can get strong one-party governments which can get things done.
Well, 53% agree with that and only 29% disagree.
So any vote would be close. And in this scenario, the nascent "progressive majority" would have been deeply divided by the LibDem deal with Cameron. Labour opinion has shifted significantly towards reform, but may harden against following a "Vote Clegg, Get Cameron" outcome. The anti-Tory broadsheets would still advocate PR if promoted by LibDems as part of a Lib-Tory alliance, but there was very little evidence last week that they command any great battalions of voters.
While his party would be nervous, and the newspapers apopleptic, Cameron could fairly easily concede a referendum and believe that it would dish the LibDems, not him.
So Cameron could concede what seem to be the central LibDem demands on electoral reform - and believe they would fail to reach their destination. The Con-Lib negotiations are certainly not likely to go any further than that, not least because of the current problems of Tory party management.
But try this heretical thought. It would be Cameron would have most to gain if he helped the LibDems to succeed. This would be the "Disraeli" option - pursued from opportunism rather than principle in extending the franchise in 1867 - of recognising pressure for reform is likely to succeed in the end, and so offering to lead it in order to derive poltical credit for its enactment.
Having failed to secure a majority, Cameron does not seem to be secure enough with his own party to try to do a Disraeli. But the Tory party's "no compromise" position may misread their own partisan interests.
Firstly, Tory support for first-past-the-post is now much stronger than makes political sense. This is based partly on an ideological right-wing antipathy to governing through compromises with other parties, but that must also be founded on the credible belief that the current system will deliver Tory majorities. If it didn't do so in the enormously favourable political conditions for the Tory opposition of 2010, how often can this be expected? Unless the trend away from two-party politics is reversed significantly, Tory leaders must now beat Labour by over 80 or 90 seats: they could not achieve that when winning majorities in 1970, 1979 or 1992. Otherwise, first-past-the-post could mean semi-permanent coalition too.
Secondly, the Tory party has less to fear than it thinks from the pluralist majoritarianism of the Alternative Vote. It preserves both the single-member link and the tendency to majority governments which are the main Conservative arguments for the current system. The fear that AV delivers permanent Lib-Labbery is unfounded. Margaret Thatcher would have won majorities under it in the 1980s; Boris Johnson was elected under a similar system. Moreover, if there was a Tory-LibDem deal, a transferable system could well have some pro-coalition bias. The LibDems regard AV as far from perfect but an improvement on the status quo, while Labour could hardly decry a system which it has just begun to advocate.
A few Tories - including Red Tory Phillip Blond - advocate AV. Douglas Carswell is the only Tory MP "out" for full PR. Yet, thirdly, the heresy can be extended to a very plausible case that it would be Cameron, not Clegg, would gain most by not just conceding a referendum to the LibDems on full PR, but by helping to get a yes vote through.
The intuitive case against this is that, with PR secure, the LibDems would abandon the right to form a more natural "progressive alliance" with Labour. That is indeed likely to happen where a Tory-LibDem alliance does not pursue electoral reform: this then provides an obvious, principled basis on which the LibDems could abandon the Tories whenever political circumstances were to allow it.
That creates a Machiavellian case for the Tories removing the cause of a later defection, and perhaps securing their own place in government too.
What if Cameron said to Clegg: I will advocate and deliver full PR (even STV) on one condition. If our coalition succeeds in delivering and implements PR, the LibDems will stand at the following election advocating the re-election of the coalition which did it. (After all, PR would make coalitions mandatory. It is certainly democratically legitimate for parties to signal their preferred coalition intentions in advance, as well as to refuse to do so).
If Clegg took such a deal (and could he realistically refuse it?) then Cameron would effectively be guaranteed re-election and eight years in power, considerably more securely than he is under any other scenario at present.
Indeed, by "founding" a realignment, an "heir to Blair" Cameron could hope to pave the way for long-term centre-right, rather than centre-left, dominance of a new system.
What would Labour do? Its only route to power would be to overtake the Tories in votes and seats under the new PR system, and then press the LibDems to switch under Clegg's doctrine of the democratic mandate. That sounds very much like a two-term project. Moreover, the party would be conflicted through anger at the LibDem "betrayal" in forming a centre-right pact, and the knowledge that a PR system would make a future coalition with the LibDems its only route back to power.
So the big winner would be Cameron - not just secure in power, but with a centrist approach locked in against an insurgency from his Thatcherite right. (That is why it will not happen. We don't know where Cameron's own instincts lie, but the deal would confirm everything his disgruntled right is saying about him to the newspapers).
This scenario of a 2015 PR election where coalition preferences of the main parties would make the government almost impossible to dislodge does demonstrate a potentially significant democratic weakness of full PR, particularly where the party system contains a decisive "hinge party". Britain's LibDems are electorally much stronger than the German FDP ever were, though the German system evolved away from a 'hinge' dynamic to develop a red-green versus black-yellow political contest. The British party system would also probably change significantly if we did have PR. Any sustained Tory-LibDem alliance could well turn the LibDems into a socially and economically centre-right liberal party, on the continental model, probably with a smaller share of the vote. Labour, Greens and perhaps new formations would compete for their current centre-left space.
So there would be existential challenges for the LibDems from success as well as failure. That is why David Cameron, not Nick Clegg, would be the main beneficiary in the (incrediby unlikely) even that the LibDems were to get the coalition + PR deal of their dreams from a most unexpected source.
Still, David Cameron has reasons to be disappointed that the Tory party of 2010 is not looking for a Disraeli.