The David Laws revelations are the first big blow to the new government and, for David Cameron and Nick Clegg, a reminder that the expenses scandal can at any moment overshadow all talk of a "new politics".
It is currently unclear whether Laws, one of the coalition's star performers, can survive. But one point in his favour is the absence of any evidence that he abused the expenses system for personal gain. Laws's claim that his motivation throughout has been not to "maximise profit", but to protect his privacy, stands up to scrutiny.
Yet the rules remain unambiguous: MPs are forbidden from "leasing accommodation from a partner". Laws's defence relies on a highly technical definition of "partner" that may not satisfy either parliament or the media.
In his statement last night he said: "Although we were living together we did not treat each other as spouses. For example, we do not share bank accounts and indeed have separate social lives."
But Laws's decision nevertheless to pay back tens of thousands of pounds "immediately" does appear to be a tacit acknowledgement of guilt. He broke the spirit, if not the letter, of the rules.
That Laws may fail to ride this crisis out is in part due to the fact that the Liberal Democrats in general, and Laws in particular, chose to make their relatively clean expenses record an election issue. Here, for instance, is an extract from the statement posted by Laws's constituency party following Sir Thomas Legg's investigation:
David has not been asked to pay back any expenses paid out to him. So far, over a third of MPs are believed to have been asked to make repayments.
I expect Laws is now regretting his decision to take the moral high ground.
And yet that this story has broken during the coalition's honeymoon period may save him. So far, he has not had to implement the sort of spending cuts that Philip Hammond once predicted would make the Chief Secretary to the Treasury "the most hated man in England".
But regardless of the verdict of the parliamentary commissioner, I would be surprised if Cameron kept him in this highly sensitive post for much longer.
Incidentally, Laws's homosexuality sheds new light on his decision to reject George Osborne's invitation to join the Conservative front bench. As Allegra Stratton's illuminating profile of Laws noted on Friday, he still had great reservations over the party's position on social affairs and personal morality.
Laws has previously told Tory MPs that he would have been one of them, had it not been for the repellent Section 28.