By Michel Rose
PARIS (Reuters) - Unknown just three years ago, and with a party only 12 months old, Emmanuel Macron has seized the presidency against all the odds. His challenge now is to govern.
To do that he must build a parliamentary majority that supports his election pledges in June legislative elections, when France's two established parties will put their huge machines to work.
Macron has at least one thing in his favor: the "majority amplifier" effect of an electoral system designed by post-war leader Charles de Gaulle specifically to maximize presidential independence from parliament.
Last week, the first opinion survey for the legislative elections showed Macron's new movement "En Marche!" could win between 249 and 286 mainland France seats in the lower house.
Even a figure at the bottom of that range would be a good outcome for him. He only needs 289 for an absolute majority, and the poll excluded 42 seats in Corsica and overseas.
It foresaw centrist and conservative parties winning around 200-210 mainland seats, the far-right National Front 15-25 and the Socialists 28-43.
"In the lowest-case scenario, En Marche would still be the largest political grouping, which would be enough to try to constitute a majority. The question would then be how and with whom," said OpinionWay's Bruno Jeanbart, who directed the poll.
En Marche is only a year old and has never fielded candidates before. Only 14 have been named so far, and at first glance a majority looks unlikely.
But that reckons without de Gaulle's amplifier - known as the "fait majoritaire" by French political scientists.
France has a two-round system in all national elections, giving voters a second chance to choose their "least worst" option. This tends to draw in votes that reinforce the legitimacy of a directly elected president, especially if, like Macron, he has a centrist program.
When they come directly after presidential elections, the legislative elections typically see a collapse in turnout among disappointed voters and a stronger turnout from those who got the president they wanted.
De Gaulle himself relied on the newly-created "Union for the New Republic" in 1958, and President Valery Giscard d'Estaing in 1978 set up the centrist "Union for the French Democracy" - like Macron, redrawing France's political map for their own purposes.
The last legislative vote in 2012 also showed the "fait majoritaire" in action. Socialist Francois Hollande garnered less than 30 percent in the first rounds of both the presidentials and the legislatives, yet came away with over 40 percent of the second-round legislative vote and, with help from 17 Green party MPs, governed with a comfortable majority.
"Macron can totally have an extremely solid majority of at least 350 MPs," said Xavier Chinaud, an electoral expert. He added that to reach that number, the president would have to employ tactics like poaching popular MPs from other parties.
The old parties will put up a fight, especially the conservative Republicans, who feel robbed of victory by the Macron factor and by the financial scandal that hobbled their candidate Francois Fillon.
Now led by Francois Baroin, they hope for enough seats to force Macron into France's fourth "cohabitation" since 1958.
Cohabitation does not have to mean paralysis, but rather that the prime minister and his camp in parliament have the upper hand over the president.
Prime Minister Lionel Jospin had the advantage in the 1990s, pushing Socialist legislation including a reduction in the working week to 35 hours under the nose of conservative President Jacques Chirac.
Economist Christopher Dembik of Denmark's Saxo Bank sees a cohabitation with the Republicans as the second most likely scenario - the first being a Macron majority that may rely on ad-hoc coalitions with smaller groupings.
The smaller groupings could come from either side of the En Marche! boundary.
The Socialists are torn between the radical left of their defeated candidate Benoit Hamon and the more centrist, pro-business branch led by former premier Manuel Valls.
There are also reports that the centrist arm of The Republicans is considering splitting off to form its own party, with which Macron might be able to work.
Macron has said he wants half of his future MPs to have no prior lawmaking experience. The first 14 candidates listed include a farmer, a hospital manager and entrepreneurs.
But being an unknown newcomer might not matter. According to Opinionway, only 38 percent of French people can name their MP.
(Editing by Andrew Callus and Andrew Roche)