Last year, Marine Le Pen spoke menacingly of the possible fallout from her trial on embezzlement charges. “Tomorrow, potentially, millions and millions of French people will see themselves deprived of their candidate for the presidency.”

After a court disqualified her on Monday from running for public office for five years, those millions of French voters are adrift and angry. France is a democracy governed by the rule of law, as the verdict demonstrated. But it is unclear how far its troubled Fifth Republic can resist an inevitable gale of political protest before the 2027 election.

Unlike President Trump, who met with convictions, indictments and criminal cases on the way to his election last year, possibly even benefiting from perceived persecution, Ms. Le Pen could find no political path past the verdict of the French legal system.

“The independence of our justice system and the separation of powers stand at the heart of our democracy,” said Valérie Hayer, a centrist lawmaker in the European Parliament. “Nobody is above the law.”

That view is certain to come under sustained attack in a global environment where questioning of the legitimacy of legal systems has become frequent — across Europe, but particularly in Mr. Trump’s United States. Mr. Trump has called for the impeachment of judges who rule against him and called them “lunatics.”

“When the radical left can’t win via democratic vote, they abuse the legal system to jail their opponents,” Elon Musk, Mr. Trump’s billionaire aide, said after the verdict.

European societies, given their history, are sensitive to the revival of far-right movements. France, like Germany, has a visceral memory of how fragile democratic institutions are and how once the rule of law goes, the way is open to dictatorial power.

“After Ms. Le Pen, the next direct target of a big political battle is going to be the rule of law,” said Alain Duhamel, a prominent political scientist. “There will be accusations that this is a government of judges, attacks on our highest court, not just from the National Rally but the center right,” he said, naming Ms. Le Pen’s party.

But, he added, “French magistrates are resolutely independent.”

Jordan Bardella, Ms. Le Pen’s carefully groomed protégé, pronounced French democracy dead, killed by the court. It is not; and to Mr. Bardella will no doubt fall the task of leading the anti-immigrant party into the election, unless Ms. Le Pen’s appeal overturns her ban in time.

At 29, he is young to aspire to the highest office, but he has demonstrated broad appeal and a near-unflappable command of detail. Just how he disentangles his ambitions from Ms. Le Pen’s remains to be seen. Up to now, they have avoided conflict.

Across Europe, the far right leaped on the court’s decision.

Matteo Salvini, Italy’s hard-right deputy prime minister, said those “who are afraid of voters’ judgment” often seek reassurance from courts’ judgment. Viktor Orban, the Hungarian prime minister, said that he stood with Ms. Le Pen.

In Moscow, Dmitri Peskov, the Kremlin’s spokesman, said, “More and more European capitals have opted for the violation of democratic norms.”

Of course, critiques of democracy from President Vladimir V. Putin’s Russia are hardly persuasive. But in this case they overlap significantly with those of the U.S. vice president, JD Vance, who in February attacked European states for trying to stifle the far right in the name of saving democracy.

Ms. Le Pen, like it or not, may now become another element in the Vance-Musk case for European democratic failure. The fact is, however, she was convicted, after prolonged investigation and on detailed evidence, of embezzling millions of dollars of European Union funds to pay party staff members with money intended for aides to European lawmakers.

Over the past decade, Ms. Le Pen led a campaign of “de-demonization,” shifting her National Rally party from its fascist antisemitic roots to an anti-immigrant mainstream party that has more seats in the National Assembly than any other.

She could now direct the party to make trouble.

The most direct means would be to overturn the centrist government of Prime Minister François Bayrou by supporting a no-confidence motion this year, in effect saying to the French people that they should be the judges and issue their verdict in a parliamentary election.

A major swing to the National Rally would not open the way for Ms. Le Pen to become president, but it would be a powerful statement.

If there is a parliamentary election, which can be held after June, Ms. Le Pen could not defend her current seat, but nothing would prevent her from becoming prime minister if the National Rally won big.

“The tribunal demonstrated its political will, not legal but political,” said Wallerand de Saint-Just, a former party treasurer who was also convicted.

Not so, said a host of centrist politicians, who have made their pride in the French legal system clear as Mr. Trump attacks a supposedly “weaponized” American judiciary.

“Madame Le Pen, whether elected or a candidate, is a French citizen,” said Sacha Houlié, a center-left lawmaker. “The law of the Republic applies.”