After Vince Cable's torturously-worded email put paid to hopes of a graduate tax, the coalition is facing the prospect of its first serious rebellion on fees. The coalition agreement allows for Lib Dem ministers to abstain from votes on higher fees, but how can Cable, whose departmental brief includes universities, credibly defend a policy that even he isn't prepared to vote for?

As one Lib Dem minister points out:

Frankly, it's going to look pretty awful for us if we're in a government that's putting forward a policy that we're not prepared to vote for ourselves. And it's going to be worst of all for Vince if he proposes something in Parliament then abstains on it.

Meanwhile, between 20-30 of the Lib Dems' 57 MPs are expected to keep their pre-election pledge to vote against any increase in fees. Chief among them is Sir Menzies Campbell, who last week told the BBC: "I will vote against any increase in the level of tuition fees. My root objection is to students being saddled with mountains of debt by the time they leave university."

Other Lib Dems, particularly those who represent university seats such as Cambridge, Leeds and Bristol, remain unambiguously opposed to any rise in fees. The creation of a US-style market in higher education -- with variable fees between different universities and courses -- is rightly seen as intolerable.

The Tories have attempted to sweeten the pill by promising that higher-earners will pay higher interest-rates on their loan -- a de facto graduate tax -- but the proposal remains unacceptable. Ed Miliband's promise to "work with anybody" who wants a progressive system of university finance -- a thinly-veiled attempt to woo disaffected Lib Dems -- only heightens the political dangers to the Lib Dem leadership.

One suspects that the Tories, like Labour in 2004, will manage to sneak the measure through Parliament. But the long-term credibility of the coalition -- and the Lib Dems -- is on the line.