By Michel Rose
PARIS (Reuters) - Emmanuel Macron's spartan presidential campaign headquarters in western Paris is abuzz with rows of young recruits busy at their laptops.
Just two months ago the offices of En Marche!, the centrist party that has shaken up France's traditional left-right political scene since it was launched last April, were empty.
It now has 200,000 members and opinion polls show that 39-year-old Macron could become France's youngest leader since World War Two by nearly 10 years after the presidential election, expected to be decided in two rounds in April and May. He is also drawing support away from the mainstream parties.
"When you're in here I really feel like we're in a young political start-up. We do things completely differently," said Maelle Charreau, a political science student at En Marche's headquarters in the French capital.
"And then there's the personality of Emmanuel Macron which appeals to all of us here, his youth but also his political courage."
His supporters see him as a breath of fresh air in a political field full of seasoned insiders such as Socialist President Francois Hollande and The Republicains presidential candidate Francois Fillon, a former prime minister.
Fillon said on Wednesday he had been summoned by a judge to be placed under formal investigation following allegations he paid his wife for no apparent work. Fillon has denied the allegations but they have dogged his campaign.
Although admirers see Macron as something different, he graduated from the elite ENA school, a training ground for many French politicians and businessmen, before working in the finance ministry and becoming economy minister for Hollande.
He was also an investment banker at Rothschild, a factor which could make him vulnerable if he up comes directly against far-right anti-establishment candidate Marine Le Pen.
But with En Marche! membership swelling -- the Socialists are down 40,000 at 86,000 since the unpopular Hollande took office, The Republicans have 200,000 and there are 83,000 members of Le Pen's National Front -- Macron has also caught the eye of established politicians from other parties.
Socialists are careful not to alienate him and he was endorsed by veteran centrist Francois Bayrou.
Funding is also rolling in. Macron has raised 6 million euros ($6.35 million) so far, the same amount the Socialist party has put behind its candidate and near the 7.5 million The Republicans have. Spending is capped by law at 22 million.
Many Macron campaigners are new to politics but are taking a grass roots approach, meeting weekly in over 3,000 committees to come up with ideas.
"We know the committees won't be writing the manifesto, but we feel maybe one measure here and there will be kept," said 30 year-old engineer Maxime Cassat, a first time member of a political party, at a recent meeting in a Paris cafe.
Mirroring the system in other parties, Macron has appointed someone in each of France's 100 sub-regional departments to manage the committees, collect grievances and ideas, raise funds and spread his message.
Laurianne Rossi, 32, a former parliamentary assistant to a Socialist senator and now a communications officer at a transport company, is one of them. In her Hauts-de-Seine district west of Paris, there are 11,000 members. Many take part regularly in workshops or campaigning, she said.
The district is a stronghold of The Republicans and the fiefdom of ex-president Nicolas Sarkozy and there are also 3,000 paying members of the Socialist party.
With polls suggesting moderates are turned off by the hard-left program of official Socialist candidate Benoit Hamon, and by the Thatcherite and socially conservative vision of Fillon, Rossi says she is also dealing with an influx of locally elected officials from both those parties.
"For some people, it's becoming an attractive badge," she said.
Macron has promised to cut public spending by about 60 billion euros, a sizeable cut but less than the 100 billion euros promised by Fillon.
As economy minister his deregulation bill facilitated Sunday shopping and opened up coach travel to compete with rail and he has made clear he wants more reforms.
But as well as drawing in supporters from both parties, his more moderate approach, to be spelled out on Thursday when he unveils his full manifesto, has won plenty of backing from economists.
"Investors think Fillon's program amounts to shock therapy, but because of France's reluctance to reform, it wouldn't work," Christopher Dembik, head of macro analysis at Denmark's Saxo Bank. "The people would take to the streets."
Since Hamon was picked as Socialist candidate in January, senior Socialist ministers including Segolene Royal, Jean-Marc Ayrault and Stephane Le Foll have been careful not to publicly criticize their former cabinet colleague Macron.
A public embrace from Hollande during a dinner in Paris last week also suggests Macron may have the support of some experienced government hands.
If he becomes president, he could then focus on the legislative elections in June.
He has said he would field candidates in all 577 French constituencies for the lower house of parliament, and has already received more than 6,000 online applications.
One Socialist MP who campaigns for Macron said about 15 sitting Socialist lawmakers had already applied for selection.
"If there's momentum to propel him to the presidency, it's fair to assume voters will follow up strongly in legislative elections," said France watcher Raphael Brun-Aguerre of J.P.Morgan.
At least 289 seats are needed for a majority in the lower house but Macron also evoked last week the scenario of a coalition ranging from "Social Democrats" on the left to "Socially-minded Gaullists" on the right.
First he has to get elected.
A Feb. 23 survey by Harris interactive, while confirming his status as favorite, showed fewer than half of Macron's supporters are sure about their choice, compared with more than seven out of 10 among his rivals' backers.
Apparent campaign miss-steps over France's colonial past and gay marriage have raised concerns he could make gaffes on the campaign trail. Some have also wondered whether the different profiles Macron is attracting would be able to work together.
"People need to speak the same language," Allianz Economist Ludovic Subran said.
(Editing by Andrew Callus and Anna Willard)