First, having annoyed many colleagues — not least in No 10 — not everyone is rushing to his defence, as they did during the suspicions that dogged William Hague.
No.10 has now given him its "full backing," but, as history shows, that can mean anything from support to sayonara. David Cameron would prefer not to reshuffle his Cabinet, that's clear. And although the newspapers will soon begin to list the people who could do — or would want — the MoD job (such as Owen Patterson, Andrew Mitchell, Paddy Ashdown, Francis Maude or Alan Duncan), a move would trigger a larger reshuffle at a bad time. But things change. The PM would probably also have preferred to hang on to Andy Coulson.
The second problem for the Defence Secretary is the fact that, at each turn, more evidence seems to emerge to suggest a impropriety of one sort of another.
A number of Cabinet minister and ministers employ people as advisers who work not out of the Departments of State but out of parliamentary offices. The cap on Special Advisers, for example, means that a Lib Dem minister in a Tory-led department is expected to rely on the Secretary of State's Special Advisers — an unrealistic scenario. So they hire people who work in their ministerial offices.
But these people are usually on the books, and do not have a conflict of interest which can undermine the minister, the government or the department. That seems to be the issue at stake in Adam Werritty's case. Labour's Good Cop/Bad Cop combination of Jim Murphy and Kevan Jones will push to see what else may lie hidden.
Fox is a formidable politician, with a large following in the party, a loyal cohort in Parliament, and friends across Fleet Street. He will not go quietly. But he is going to have to put up a fight this time.