Gordon Brown's future as Prime Minister hangs on three things: Labour's fate in the council and European elections; the reshuffle he will attempt; and the attitude of Alan Johnson.

Brown's tenure as PM has been Sisyphean in its return to the bottom of the hill. It is a merciless job. Despite the occasional moments where the 'not flash, just Gordon' persona clicked with events (the financial meltdown; the calm response to the flooding and the terrorist attacks), he has struggled with a more charismatic and media-friendly opponent in PR man David Cameron.

The scandal over MPs' expenses produced a fast response from Cameron. Brown's seemed dilatory, and although the affair has hurt the Conservatives' rebranded 'nice' image, Brown has struggled to make the most of the crisis. It is totally unclear what his proposals for reforming Parliament are.

The recession has made people angry and scared. They are in a mood to kick somebody - anybody - and politicians are nearby and unloved. The impact of European politics on everyday life feels remote to most voters; council elections affect how often your bins will be collected and roads resurfaced. The protest vote may be more obvious in the European poll results on Sunday.

However, if Labour comes fourth after the Tories, Lib Dems and UKIP, the nascent Westminster rebellion of Labour MPs, given momentum by Hazel Blears' departure, may not leave Brown the time to finish reshuffling his Cabinet.

The reshuffle is another challenge, if he gets that far. For the first time, the reliable Michael White seems to indicate that he now thinks Brown's exit is at least as likely as his grim clinging on. White's analysis is strong on the devastating consequences for government of an enfeebled staggering-on.

If an enfeebled Brown tries to go on, cabinet ministers who don't want to be moved or dropped could rip his plans to shreds. They could also add some name-recognition impetus to the 'Gordon must go' contingency.

Health Secretary Alan Johnson yesterday made another protestation of loyalty to Brown, telling the BBC that Brown "is doing the job and there is absolutely no one who could do that job better."

Would he stand? Not against Brown, it would seem. But if Brown falls on his sword, Johnson will stand - and could win.

On the back of generally good performance by the NHS, Johnson has made health a much lower-key political issue. As Tom Smith observed last year, Johnson must be the only minister who is delighted when his speeches are not reported on the front pages, if at all. (Once the inevitable spending cuts come, the NHS's low political profile will change.)

Yet it has been hard to discern what 'Johnsonism' might be politically, other than a consensus-building legacy from his past as a trades union fixer.

Johnson's affable persona and lack of committed political enemies would count strongly for him as a candidate to be PM. His previous statements about not being up to the job of PM would be a hostage to fortune. Looking at a Brown future, however, many Labour MPs might feel they could give that hostage, in the hope that Johnson could connect with the electorate.

So there is a strong chance that Johnson will travel to the NHS Confederation in Liverpool next week as a Prime Minister in waiting. Would we see Lord Darzi as his health secretary?