One of the most extraordinary French presidential election campaigns in recent history took a sinister final twist with claims that frontrunner Emmanuel Macron was the target of a “massive and coordinated hacking attack” just hours before polls open on Sunday.

Hacked campaign documents, internal emails and financial data were posted online anonymously along with papers Macron’s team said were false, just before midnight on Friday, the official end-of-electioneering deadline.

The leak cast a long shadow over the legal “election pause” during which Macron and his far-right rival Marine Le Pen are banned from making any statement until polls close at 8pm French time on Sunday. France’s electoral commission warned that publication or republication of the documents could be a criminal offence. The commission urged media and citizens “not to relay” the leaked documents “in order not to alter the sincerity of the vote”.

Just before the deadline, Macron’s En Marche! team said fake documents were mixed in with papers showing legitimate campaign activities in order to “spread doubt and disinformation”.

“This is not just a simple hacking operation but a real attempt to upset the French presidential election,” it said.

En Marche! says it has been the victim of repeated hacking during the campaign. It blames groups backed by the Kremlin, which supports Le Pen. Moscow has denied any involvement. On Thursday, Macron launched legal action against “fake news and lies” after documents suggesting he had an offshore bank account in the Caribbean appeared on the internet.

The leak is reminiscent of the hacking of Hillary Clinton’s campaign manager John Podesta’s emails during the US election campaign. White House experts have blamed a Russian group of hackers.

Le Monde said the latest leaked documents were quickly spread by France’s far right on Friday. On Facebook, the Front National vice-president, Florian Philippot, one of Le Pen’s closest advisers, published a WikiLeaks link to the En Marche! documents two minutes before the midnight deadline, adding: “With MacronLeaks are we learning something investigative journalists have deliberately hushed up?”

At such a late stage, the hack seems unlikely to influence the outcome of today’s election. With the final Ipsos poll showing victory for Macron with 63% of votes and Le Pen 37%, the En Marche! candidate will be hoping to put a scrappy election behind him and look forward to an in-tray which looks intimidating to say the least.

If Macron becomes France’s next – and youngest ever – president on Sunday, he will have come from nowhere. Three years ago, when he was named as President François Hollande’s economy minister, journalists were writing “Who is Emmanuel Macron?” articles.

If victorious, he will be charged with taking France out of an economic and social crisis that has left the country so divided and despondent that millions of voters turned to the political extremes. His room for manoeuvre, however, not only depends on winning against Le Pen but on winning an overwhelming mandate.

Macron, who has never held an elected post, has promised change, a new moral dawn in political life, and a bright new future. He will have to deliver quickly with, to date, no formal political party. His movement, En Marche!, has never fielded a single candidate in any election, local or national.

In 2012 Macron’s mentor, the newly elected François Hollande, promised he had “set a course” for France. The honeymoon period was brief. Within months, critics were portraying him as a “pedalo captain” going round in circles (an insult invented by the hard-left candidate Jean-Luc Mélenchon).

Macron, already accused of being Hollande II, will want to avoid that, but needs to move fast between now and the June legislative elections if he is to secure a majority of seats in the national assembly. If he cannot achieve that, there is a danger that France will be politically paralysed, making the two-round general election at least as important as the presidential vote.

Pascal Perrineau, head of the Science-Po university’s political research institute, said that, while the electorate’s reflex after a presidential vote was to give the new leader a parliamentary majority, Macron would find it a challenge to rustle one up.

“Macron has no MPs. If he wins, he will have to organise a parliamentary majority from scratch in a few weeks,” Perrineau said. “So, you are elected, but the question is, who will you work with? If you have no parliamentary majority, you cannot govern; it’s a dead election. Both Macron and Le Pen are amateurs who don’t understand the logic of parliamentary life. Alone, he is nothing. Politics is an entire career, people are not going to rally to him just because he has an extraordinary smile, young blood and blue eyes. There will have to be some serious negotiations.”

Under France’s constitution, the president is supreme political leader and can choose a prime minister, who then recommends government ministers, all of them normally from the president’s party. If Macron obtains an outright majority from a standing point of zero seats, he will be able to enact policies which are Europe-friendly and economically liberal. If he faces a hostile majority in the lower house of parliament and is forced to appoint a prime minister outside his party, he will face a period of what is known as “cohabitation”, meaning he can do little in the face of constant opposition. A third possibility is that En Marche! could have the biggest group of MPs but not a majority, leaving a Macron government with only limited room to manoeuvre but not entirely stymied.

Emmanuel Macron has been accused of being too similar to his mentor, Françoise Hollande. Photograph: Alain Jocard/AFP/Getty Images

Working in Macron’s favour would be the current disarray in both the Socialist party and the conservative-right Les Républicains following their candidates’ first-round defeat. Around 200 MPs are also standing down because they will no longer be allowed to accumulate elected positions and prefer to maintain local roles. Macron has already picked up support from some political heavyweights from across the political spectrum, but would not want too many familiar political faces to avoid charges of “plus ça change”.

In January, Macron announced En Marche! would field candidates in all 577 parliamentary constituencies. He wants half to come from civil society or elected local authorities. All must have a clean criminal record, renounce current political affiliations and campaign as an En Marche! candidate. At least half must be women.

Andrew Knapp, emeritus professor in European studies at the University of Reading, believes that Macron probably faces a “protracted war of manoeuvre to get legislation through, typically picking different majorities for different issues”.

“All will depend on the size of the pro-Macron forces in parliament. That in turn will depend on his showing on 7 May. If Macron were to do significantly better than the 62% suggested by the polls, the resulting dynamic could bring his supporters close to a majority six weeks later. Anything less than 60% would be seen as a relative defeat. For that reason, if no other, the result of the presidential second ballot should be treated as anything but a foregone conclusion,” Knapp writes on the UCL Constitution Unit blog.

According to an OpinionWay-SLPV analytics survey, Macron could gain between 249 and 286 seats, the centre- right 200 to 210, and the Front National 15 to 25. On the left, the Socialists would score the worst result in their history with between 28 and 43 seats and Mélenchon would get only six to eight, it predicts. Political experts, however, have described the poll as highly speculative, given that it is based on intentions to vote and En Marche! has only 14 declared candidates so far.

Aware that many are not voting for him but against Le Pen, a triumphant Macron will want to avoid squandering his popularity with silly mistakes or any triumphalism. After the first-round vote he dined at a chic restaurant and announced he would find a role for his wife Brigitte, both of which sparked criticism.

The success of an eventual President Macron will also depend on how many angry and disappointed FN and hard-left Mélenchon supporters take to the streets. And what he does when they do.

WHO WILL BE MACRON’S PRIME MINISTER?

Sylvie Goulard, 52

From the centrist MoDem party whose leader François Bayrou was one of the first political heavyweights to rally to Emmanuel Macron, the Marseille-born Goulard is vehemently pro-Europe, a supporter of greater federalism and a graduate, like Macron, of the elite École Nationale d’Administration. Fluent German speaker. Was part of a team negotiating the reunification of Germany in the 1990s and arranged for Macron to meet the German chancellor, Angela Merkel.

Anne-Marie Idrac, 65

Transport secretary in the 1990s, then president of the state-owned transport companies RATP (Paris region) and SNCF (railways). Also a graduate of ENA. Belongs to the Nouveau Centre party.

François Bayrou, 65

Veteran centrist and leader of MoDem, whose early support for Macron boosted the election campaign. A former history teacher, Bayrou was a candidate in the 2002, 2007 and 2012 presidential elections.