Who will be the first Lib Dem member of cabinet to quit this coalition during the course of this "fixed-term" parliament? I know we've already had one Lib Dem departure: the former chief secretary to the Treasury David Laws, of course, resigned after just 17 days on the job. But he didn't really have a choice.
I'm talking about the possibility of a so-called principled resignation by a minister -- if you'll allow me to include the world "principle" in the same sentence as "minister". There are five Lib Dem members of the cabinet -- Nick Clegg (Deputy Prime Minister), Vince Cable (Business Secretary), Danny Alexander (Chief Secretary to the Treasury), Chris Huhne (Energy Secretary) and Michael Moore (Scottish Secretary).
"Calamity" Clegg ain't going anywhere any time soon. As the leader of the Lib Dems, he can't really resign from the cabinet without bringing down the whole coalition government. And so he's hitched his own political future, as well as his party's, to this Lib-Con coalition deal and he now has nowhere else to go. (I'm told John Denham spoke for the entire shadow cabinet when he told the Fabian Review last month that Labour would demand the head of Nick Clegg before doing any deal with the Liberal Democrats in the future.)
Danny Alexander, meanwhile, has become the face of the cuts. I can't see him going either. And Michael Moore? Would anyone notice if he left the cabinet? I'm not sure anyone really noticed when he joined it, after his Lib Dem precessor at the Scottish Office, Alexander, went off to replace Laws at George Osborne's side in HM Treasury.
Then there are Huhne and Cable, both former Labour men who are often described as being on the left of their party. A few months ago, I'd have put my money on Huhne, but the way in which he survived the revelations of his affair, and the subsequent break-up of his marriage, including the repeated attacks from the Daily Mail, suggests he is a man who has no plans to quit front-bench politics. It has become clear, listening to Ed Balls and Ed Miliband describe how keen Huhne was during the coalition negotiations with Labour to abandon the Lib Dems' manifesto pledge to delay spending cuts, that he has far fewer ideological objections to Osbornian austerity than some of us might have assumed in the not-too-distant past.
So that leaves St Vince of Cable, who keeps being humiliated by his Tory coalition partners. First, Cameron and Osborne kowtowed to Tory backbench concerns by limiting the rise in capital gains tax to 28 per cent, after the Business Secretary had accused the likes of David Davis and John Redwood of "reinventing the wheel" on CGT. Then, after Cable announced his interest in a graduate tax, a "senior Conservative source" promptly briefed the BBC that the government would reject such a proposal.
No wonder Vince looks so grumpy and down, or, as I wrote in a column a few weeks ago:
Vince Cable, the Business Secretary (who, in the words of one senior Labour politician who knows him well, is "semi-detached" from the government), is doing his best impression of Walter Matthau in Grumpy Old Men. He is said to mope around the Department for Business, Innovation and Skills (BIS), while some Lib Dems believe he is setting himself up as a restraint inside the cabinet on his old enemy, George Osborne.
In a "candid" interview in today's Sunday Telegraph with Patrick Hennessy, Cable seems to confirm my view:
"People sometimes ask me, 'Are you having fun?' " he says. "No! It's hard work and it's tough, but it's important."
This mantra of fairness is central to his beliefs. He is often cited as the Lib Dem most likely to quit the cabinet on a policy issue, so what are his "red line" issues that might provoke such an exit?
"I worked for some years to get us committed in our party to what we call fair taxes, lifting low-paid people out of tax, we got that in the coalition agreement and it was in the first Budget. So I'm content that that's being carried forward."
Later on Mr Cable returns to the subject of "fair taxes", when asked what he would consider a success after five years as Business Secretary. He even goes much further than Labour ministers ever dared by using the "R" word (redistribution) and spelling out exactly what he means -- "a tax system that means people at the bottom end of the scale pay less and at the top end of the scale pay more".
"Fair taxes" are not what we've got so far from this coalition government, with its "regressive" (© Institute for Fiscal Studies) emergency Budget in June. It's not fair to raise VAT, which hits the poorest hardest, while cutting corporation tax on the banks and leaving bankers' bonuses untaxed. And I have no doubt that the coalition's tax-and-spend changes are only going to get more unfair. How long will Vince Cable be able to stay in such a government while sticking to his self-professed social-democratic principles? Not that long, is my guess.