The real impact of Norwich will be on public's view of party leaders

This by-election is the first tangible electoral consequence of the expenses fiasco in the wake of Ian Gibson's demise. It will send MPs off on their summer break with a litmus test of public opinion, giving them something to worry about in this last summer before a general election.

For the Conservatives, a victory would do nicely to confirm the indicators of an upward trend towards general election victory.

"Cameron hasn't sealed the deal" is the self-consoling Labour line, which is probably true: he has no sustained leap in the polls à la Blair before 1997. But then he doesn't need to so long as he is getting the voters ready to sign on the dotted line, come the real polling day.

Norwich is not a certain outcome. It's a hard-fought seat which has swung between the Tories and Labour and has one of those enticing majorities around the 5,000 mark that can slide either way.

The Government, however, seems to attract bad luck as well: its candidate has been assailed by swine flu. Next omen: four horseme * and a touch of fire and flood. But remember that by-elections can be surprising. Labour rose from the dead last year in Glenrothes, when it learned the lessons of Glasgow East and got its act together in the local campaign.

The real impact of Norwich will be on the perception of the parties' respective leaderships and their dynamic towards the election. That will be influenced either by a surprise Labour win - Gordon does a Lazarus - or by a Labour slump into another third place after the local and European election humiliation.

Mr Brown is over-associated with failures, and while no late-life government does brilliantly in by-elections, he needs to show that his strengths - consistency of message, seriousness of purpose - are capable of holding the line against a Tory advance. His problem is that any major setback - third place tonight, or a failure to pick up in the polls after Labour conference - are likely to unleash calls for his replacement. This may be a dwindling prospect, but it is not a negligible one.

If he looks like a man who, whatever his virtues, always loses in the end, the clamour for Alan Johnson to step into the breach will grow.

The Home Secretary, a mixture of agreeable Jack the Lad and canny trade unionist made good, is the only candidate warring Labour factions can agree on. He has carefully positioned himself as the man who is not seeking office. If Mr Brown begins to look irretrievably disaster-prone, Mr Johnson could still find greatness thrust upon him.

To stave off any such intervention, the Prime Minister must maintain a sense of momentum towards the election. The Brown recovery strategy is simple: wait for signs of economic recovery, choose the moment and declare that his emergency spending package and pro-active approach to the crisis is vindicated.

Mr Cameron's strategy will be to say that this is rot: and that his more prudent approach to public borrowing and a stringent tone on public spending are remedies the electorate will ultimately respect. But the contradictions inherent in this position and his earlier "sunlit uplands" approach are increasingly apparent.

The growing tension highlighted in the New Statesman this week between the Tory leader and the London Mayor are partly those of personal pride and ambition.

They are also grounded in the ready assumption that money for projects like Crossrail would be ringfenced under a Tory government. Right now, very little is in that magic ring at all on shadow chancellor George Osborne's calculations.

Mr Johnson might reasonably ask why something as ill-accounted for as international development is sacrosanct (a hangover from the days of aping New Labour) and Crossrail is not.

But what Mr Cameron is anxious to avoid is a spat which pits the leadership against the great London populist.

Mr Johnson is naturally keen to extend his powers as Mayor and have a degree of independence - not least because he is an individualist, not a team player.

Any such tensions towards and election will irk the Cameron-Osborne duopoly who really run the Tory Party, whether the Mayor likes it or not.

If the Conservatives win Norwich tonight, Mr Cameron will have put his seal on the political year and even his most turbulent insiders will have to accept that.

If Labour loses, Mr Brown has another summer of discontent ahead as he strives to shake off the reputation as the man who just can't win. Norwich could never have expected to matter quite so much.