After months of skirmishing, the French presidential candidates are finally under starter's orders and the real contest can begin. That the campaign seems to have been going on forever, though, should not be allowed to obscure the significance that the outcome will have, not just for France, but for Europe and for Britain.

On the face of it, French voters are contemplating a straight, and very traditional, choice between left and right. The incumbent President, Nicolas Sarkozy, is an unapologetic advocate of the free market (albeit with French characteristics), who combines a strong streak of libertarianism with old-fashioned Gaullism. His chief opponent, the Socialist, François Hollande, started out as an almost accidental candidate – the beneficiary of Dominique Strauss-Kahn's continuing woes – but impressed doubters early on, showing an oratory and passion he had hitherto concealed. He, too, has come across as an old-fashioned politician, of a statist variety.

There are small-scale dynamics to be watched: a potential increase in the National Front vote in the wake of the recent shootings in south-west France, which could cost Mr Sarkozy crucial support, and the unpredictable appeal of the far-left candidate, Jean-Luc Mélenchon, who could take support from Mr Hollande. The centrist, François Bayrou, seems once again not to be making a mark.

But a contest that once seemed to be Mr Hollande's to lose – with France's stagnant economy and the euro's difficulties making incumbency a liability for Mr Sarkozy – has now tightened as the President mobilises his formidable campaigning skills. Incumbency could even turn out to be a plus, as Mr Hollande's shortage of top-flight experience is exposed. Disgracefully adept at playing on French security fears when an election is at stake, Mr Sarkozy is not the loser yet.

With many French voters yet to make up their mind, the result will turn on which they regard as the lesser evil: more of the medicine prescribed by Mr Sarkozy, or a retreat to the bosom of a cash-strapped state. The first would probably mean more of the same uneasy quest for Europe-wide technocratic solutions. The second could turn the politics of Europe upside down, with leftward change suddenly seen as possible elsewhere. Whatever happens on 22 April, no one should be in any doubt. This is a French election that really matters.