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THE ELECTION of Donald Trump means that the future of the democratic West, as it as has existed since 1945, may now depend on a string of elections in Europe during the next few months. A Dec. 4 constitutional referendum in Italy may determine whether its reformist center-left government crumbles and succumbs to the populist right; elections in Austria and the Netherlands could bring far-right leaders to power. Perhaps most important, a French presidential election in the spring is expected to devolve into a contest between a representative of the country’s traditional political establishment and the nationalist Marine Le Pen, who promises to overturn it. Her victory and the arrival of Mr. Trump could trigger the collapse of NATO or the European Union and dissolve the more diffuse community of nations that have shared liberal democratic values.

Observers far from Paris consequently have been paying close attention to the two-round primary election of France’s center-right Republican party. The first vote, last weekend, produced another shock to the polling industry: The winner was not Alain Juppé, the most centrist of the candidates, or former president Nicolas Sarkozy, who positioned himself as an anti-Islamist hard-liner, but former prime minister François Fillon, who neatly inserted himself between them. Mr. Sarkozy was eliminated from the running, and Mr. Fillon, who won 44 percent of the vote, was judged to be a heavy favorite to win this weekend’s second round. If he does, he will instantly become the front-runner in the general election, expected to face Ms. Le Pen in a second round.

That would be cold comfort for American internationalists. Mr. Fillon, who served as Mr. Sarkozy’s prime minister for five years, shares Mr. Trump’s admiration for Russian President Vladi­mir Putin as well as the president-elect’s view that the West should join Russia’s alliance with Syrian dictator Bashar al-Assad. That puts Mr. Fillon in the same boat as the stridently pro-Moscow Ms. Le Pen. Both are also anti-American: Mr. Fillon has promised “a serious confrontation with the United States” over its alleged domination of the European economy and opposes a transatlantic free-trade treaty. Domestically, he is a social conservative who opposes gay marriage.

Mr. Fillon differs from Ms. Le Pen in not favoring France’s withdrawal from the euro or a referendum on its E.U. membership. And he seems to have stood out with right-wing primary voters for his promises of radical — by French standards — action to liberalize the economy. Welcoming comparisons to Britain’s Margaret Thatcher, Mr. Fillon says he would slash state spending and the civil service, repeal a wealth tax, raise the retirement age and end a mandatory 35-hour workweek. It’s medicine that virtually everyone in the French political elite now sees as necessary to revive growth and reduce an unemployment rate that over the past 20 years has averaged 9.3 percent. But few have been willing to face the backlash from powerful unions and disgruntled citizens.

Mr. Fillon’s readiness to fight for such change separates him from the establishment, seemingly a good step in any Western democracy at the moment. But it may make him more vulnerable to Ms. Le Pen. If left-leaning candidates are eliminated in the first round of the presidential vote, their supporters may balk at Mr. Fillon or be tempted by Ms. Le Pen’s populist nostrums. That means that France’s election may end up being as unpredictable as that of the United States — and for the Western alliance, potentially fateful.

Read more on this issue: Jim Hoagland: America’s internationalist ideal is dying in Europe Anne Applebaum: Has Europe found an antidote to authoritarianism? E.J. Dionne Jr.: Trumpism: Made in Europe