Margareta Pagano: If Hollande takes victory he will need to win Merkel over too

Angela Merkel arrives at passport control at Charles de Gaulle airport in Paris. The guy at the immigration desk fires off the usual questions: "Name?" "Merkel," she replies. "Nationality?" "German." "Occupation?" "No, just here for the day."

OK, I know it's pretty tasteless and a new play on an old joke. But the fact that this mischievous variation is doing the rounds shows how successful François Hollande has been in stepping up the tempo between France and Germany as he edges closer to becoming the next French President.

Even for an election campaign, Mr Hollande's criticism of Germany has been extraordinarily robust. His pledge to smash Chancellor Merkel's fiscal pact, agreed last December to save the eurozone, has been a central plank of his campaign to oust the incumbent, Nicolas Sarkozy.

In fact, Mr Hollande is proving something of a smoothie-chops – promising to everyone all that they want to hear. To the international investors that own France's debt, he's promised to stick to the deficit reduction plan – and wipe it out by 2017. He's told the French public, nervous about spending cuts, that 60,000 new teachers and 5,000 new police officers will be hired and 150,000 new jobs for the young created, while the unions have been promised minimum pensions reform.

You can see why Mr Hollande's pitch looks smart – his claims of support from left and right across Europe on breaking up the austerity pact look more astute by the day as first the Dutch and now the Austrians are retreating from the Merkel position. In Greece, the most likely outcome of this weekend's election is a fractured coalition – mainly because of the public's anger towards both the two big parties for selling out to the Germans.

Mr Hollande is not the only one who has been firing up passions along the old Franco-Prussian fault-line. One in five French also voted for Marine Le Pen's Front National – anti-euro and pro-French franc. It now looks as though Mr Hollande will pick up most of the FN vote, as many of those who voted for Ms Le Pen detest Mr Sarkozy, whom they claim cost them votes at the 2007 election.

So, unless Mr Sarkozy performs a miracle, Mr Hollande will romp home as the first Socialist president in more than two decades. But Mr Hollande's victory may prove short-lived – as little as a month, in fact. For everyone seems to be forgetting that France's parliamentary elections are on 10 and 17 June, and the predictions are that Mr Sarkozy's UMP party will win another big majority in the National Assembly. Before parliament was dissolved, the UMP had 313 seats, the PS – the Socialists – 186 and the Centre party 22 seats.

It's become orthodoxy among commentators that Mr Hollande has done so well because he's put such a clear line between himself, as the populist cheerleader of anti-austerity, and the mores austere Mr Sarkozy, otherwise known as one half of Merkozy.

Yet all the French people I speak to reckon the UMP will win another majority. They say much of the voting for Mr Hollande is tactical – as much due to voters' disdain for Mr Sarkozy's messy divorce and celebrity marriage as for his austerity reforms and cosying up to Mrs Merkel.

Both Messrs Hollande and Sarkozy have promised to cut the budget deficit to 3 per cent of GDP next year, but how they get there differs. Mr Hollande has promised savings of €100bn (£81bn) over the period: 50-50 between tax increases on high earners, big companies and banks, and cuts to spending; even promising to make French expats pay for their lycées overseas. Mr Sarkozy is more radical, arguing for savings of €115bn to balance the budget by 2016, made up of €75bn in spending cuts and €40bn in higher taxes.

Obviously, Mr Hollande will try to get as much leverage in the first weeks of the honeymoon but after that it will be tough negotiations with the centre-right – but also with his more left-wing supporters who could be even more troublesome.

If Mr Hollande moves into the Élysée Palace, his first visit should be to Berlin to persuade Mrs Merkel that austerity alone is not enough, the treaty needs modifying. Grown-up debate is now needed about how to use the EU budget to improve growth, and how to boost investment.

To use his victory well, he should persuade Mrs Merkel that Europe needs a modern-day Marshall Plan; one that shifts the mood from pessimism to optimism. It's in Germany's interest too, as it needs a thriving Europe to ensure its survival.