Dogged by the migration crisis and the traumatic business of Brexit – to name just two current, existential challenges to their project – those who run the European Union felt they had enough on their plates before Donald Trump seized the White House.

News of his triumph broke on Europe, as had that of the British vote to leave the European Union on 23 June, in defiance of opinion pollsters and the assumptions of political elites that maintained that the world’s most advanced democracy could never deliver such a blow to the established order. Then it did.

In EU capitals, where they had preferred to dismiss Brexit as a one-off revolt by the union’s most difficult member, Trump’s election prompted the same elites to question their easy assumptions and entertain, for the first time, the impossible.

On Wednesday morning in Paris, a senior member of the French establishment, with experience of service at the highest levels in Europe and Washington, said of his country: “I wasn’t worried about what would happen here. But now I am concerned. I think we have to be.”

The question was “what next, where next?”

France is heading towards a presidential election next spring in which the populist, anti-EU leader of the Front National, Marine Le Pen, is widely expected to reach the second round runoff, probably against the veteran centrist Alain Juppé. He is the clear favourite to enter the Élysée. But the speed with which the Front National pounced on events in the US to suggest otherwise fed the sense of unease not just in Paris, but in Brussels, Berlin and elsewhere. “Today the United States, tomorrow France,” tweeted Le Pen’s father, Jean-Marie, the founder of the party.

A former prime minister between 1995 and 1997, Juppé is hoping to appeal to those who feel let down by recent presidents and want a more responsible head of state after the chaotic tenures of François Hollande and Nicolas Sarkozy. But Juppé is an establishment figure from the old order, and will be portrayed as such by Le Pen. Just as Hillary Clinton was by Trump.

Big names in the French political world are now saying nothing can be regarded as certain any more and Le Pen cannot, and must not, be dismissed as the inevitable runner-up. “Reason no longer prevails since Brexit. Mrs Le Pen can win in France” concluded former prime minister Jean-Pierre Raffarin.

For the European Union such an outcome – Le Pen winning – would be far, far worse than Brexit. Brexit is containable. A France conquered by an anti-EU presidential candidate is not.

Everyone agreed last week that her winning would destroy the EU. “It would be cataclysmic, existential, the end,” said one EU diplomat.

In Berlin, Stephan Mayer, a Christian Social Union (CSU) MP in the Bundestag and his party’s home affairs spokesman, declared that, if Le Pen took France out of the euro and the EU, the European project would be done for.

Norbert Röttgen, chairman of the foreign affairs committee in the Bundestag, and one not prone to dramatic overstatement, said countries at the heart of the EU integration process could no longer regard themselves as necessarily immune from populist movements. “What we have to take into account is that disruptive things can happen and the unthinkable can happen, so we should not take it for granted that Le Pen cannot win,” he said.

Philippe Juvin, a French MEP who is helping Sarkozy with his fresh bid to become conservative candidate for the presidency, said that the search for votes had to be aimed at the same kind of white working-class voters in France who felt left behind as those who had backed Trump in the US.

“We have to stop saying that these people are wrong and listen to them,” he declared. The danger from Le Pen “does clearly exist”, he said, adding that France had a history of toppling establishment power. “You do not understand anything about the French if you do not understand that we are revolutionaries. Remember also that only 10 years ago we voted down the EU constitution in a referendum.” Something just as big could happen again – bigger, in fact.

France is not the only electoral concern for Europe’s leaders as they try to hold together the union and hold back populist surges. Next year voters in the Netherlands will go to the polls, with Geert Wilders of the far-right Dutch Freedom party a threat, and also in buoyant mood after Trump’s win. “Politics will never be the same again,” Wilders said on Wednesday. “What happened in America can happen in Europe and the Netherlands as well.”

Freedom party leader Geert Wilders, who will challenge the Dutch establishment next year, insists politics will never be the same again. Photograph: Bloomberg/Bloomberg via Getty Images

Before the big election year of 2017 – which will also see German elections next September – Austrians will vote early next month in a presidential election that could see Norbert Hofer of the Freedom party become the first far-right head of state to be freely elected in western Europe since 1945. And on the same day, 4 December, there will be a constitutional reform referendum in Italy in which prime minister Matteo Renzi has staked his future and is under threat from Beppo Grillo’s left-wing Five Star Movement, while in Poland and Hungary right-wing nationalists are growing in prominence.

Trump’s victory has shaken the EU in other ways, too. Its leaders now talk very tough about Brexit, saying they will deliver a hard deal on the British if, having decided to go, Theresa May demands membership of the single market, Europol and a slice of EU research money without submitting to European rules, the jurisdiction of EU judges and paying into EU budgets. Much of the tough rhetoric is coming from Paris, where the worry is that, if the UK is seen to be given too favourable an exit deal, that will be yet more encouragement to Le Pen and her attempt to cause a political sensation in Europe on the scale of Trump’s in the United States.

A year of political unrest

4 December: Italian constitutional referendum

Voters go to the polls in a referendum called by prime minister Matteo Renzi to ask if Italians approve of reforms on which he has staked his political future. The anti-establishment Five Star Movement, under its founder Beppe Grillo, right, currently heads the polls. Grillo hailed Trump’s victory as a vindication of his own maverick stance. Renzi has said he will resign if he does not win.

4 December: Austrian presidential election

The Austrians are re-running a presidential election that could see Norbert Hofer of the rightwing, anti-immigrant Freedom party, become the first head of state from the far right to enter office in an EU state since 1945. The result of the first election in May, in which Hofer, right, came within 31,000 votes of winning, was scrapped due to irregularities in counting the postal ballots.

15 March 2017: Dutch elections

The Freedom party of rightwing populist and anti-immigration campaigner Geert Wilders, right, is neck-and-neck in opinion polls with prime minister Mark Rutte’s liberals. Wilders, who attended the Republican party convention earlier this year, has imitated the Leave campaign in the UK referendum, saying Dutch voters must “take back their country” on election day.

23 April/7 May 2017: French presidential elections

The first round of voting will be on 23 April and the leader of the anti-EU Front National Marine Le Pen is widely expected to progress to the decisive runoff. Then she is most likely to face veteran centrist Alain Juppé, a former prime minister, in the decisive vote on May 7. Juppé is favourite in a country that remains broadly pro-EU, but there is nervousness that a shock is now not impossible.

September or October 2017: German elections

Germany is the least likely country to see a populist leader. However if the euro-sceptic (by German standards) Alternative für Deutschland, led by Frauke Petry, right, beat their current poll ratings, which are in the low teens, by a big margin that in itself would constitute a shock. Angela Merkel has not announced if she will stand again but her CDU/CSU is well ahead, albeit damaged by her approach to migration.