This month, President Biden, flanked by President Emmanuel Macron of France, stood on the Normandy bluffs to commemorate the young men who clambered ashore 80 years ago into a hail of Nazi gunfire because “they knew beyond any doubt there are things worth fighting and dying for.”

Among those things, Mr. Biden said, were freedom, democracy, America and the world, “then, now and always.” It was a moving moment as Mr. Macron spoke of the “bond of blood” between France and America, but just a few weeks later, the ability of either leader to hold the line in defense of their values appears more fragile.

The United States and France — pillars of the NATO alliance, of the defense of Ukraine’s freedom against Russia and of the postwar construction of a united Europe — face nationalist forces that could undo those international commitments and pitch the world into uncharted territory.

A wobbly, wavering debate performance by Mr. Biden, in which he struggled to counter the dishonest bluster of former President Donald J. Trump, has spread panic among Democrats and raised doubts about whether he should even be on the ticket for the Nov. 5 election.

Uncertainty is at a new high in the United States, as well as in a shaken, startled France.

The country votes on Sunday in the first round of parliamentary elections called by Mr. Macron to the widespread astonishment of his compatriots. He had no obligation to do so at a time when the far-right National Rally, triumphant in recent European Parliament elections, seems likely to repeat that performance and so perhaps attain the once unthinkable: control of the French prime minister’s office and with it, cabinet seats.

“Since Normandy, the Biden debate and the National Rally score have been severe blows,” said Alain Duhamel, a prominent French author and political analyst. “Mr. Macron’s decision was an unwinnable bet, and so a dangerous bet, now headed, it seems, for failure.”

Mr. Macron will, barring a shock resignation, remain as president after the election and has warned repeatedly of the very international scenario that now seems more likely. In essence, he has argued that an unreliable America, where the return of Mr. Trump was plausible, necessitated the creation of a “Europe power.”

It was notable in the debate that Mr. Trump did not deny that, if elected for a second term, he might withdraw the United States from NATO. Last month in Dresden, Mr. Macron said that a “more independent, more sovereign Europe able to defend itself and survive against all threats” was now a paramount need, given that “America’s priorities are sometimes elsewhere.”

The problem for Mr. Macron now is that his capacity to forge a Europe of integrated industries, greater defense capacity and sweeping military integration may well be curtailed, or even eliminated, if he has to govern with Marine Le Pen’s euroskeptic National Rally.

A failed gamble on the two-round election that finishes on July 7 would most likely leave him a reduced figure, unable to deliver on his bold international plans. The latest Ifop-Fiducial poll this week gave Mr. Macron’s party and its allies just 21 percent of the vote. The National Rally was in a comfortable lead at 36 percent, and the New Popular Front group of parties ranging from the socialists to the far left at 28.5 percent.

A group of 170 anonymous French diplomats warned in the newspaper Le Monde last Sunday that “our adversaries will view the victory of the extreme right as a weakening of France” and an invitation “to aggression against Europe, including militarily.”

They did not mention Russia by name, but their message was clear enough. The Kremlin, which long maintained close ties with Ms. Le Pen, the perennial far-right presidential candidate, and her party, has said it is “following attentively the progression of the forces of the right.”

The National’s Rally’s ascension to high office, by no means certain, would mark a turning point in Europe.

France, with Germany, is the cornerstone and motor of the European Union. It is not Prime Minister Viktor Orban’s anti-immigrant Hungary, an irritant to the European project, but in the end a marginal one. If France turns against a united Europe, the possibility of a broader unraveling of the 27-nation union grows because its core begins to dissolve.

The fact that Chancellor Olaf Scholz of Germany also finds himself weakened by a faltering economy, a fractious coalition and a rising far-right party constitutes a further challenge for Europe.

Jordan Bardella, Ms. Le Pen’s 28-year-old protégé who would probably become prime minister with a National Rally win, has been at pains to indicate that he will not upend France’s international commitments. He vowed continued support for Ukraine, but he has drawn a red line at French forces on the ground or the provision of long-range missiles capable of hitting Russia.

Mr. Macron has said “nothing should be ruled out” with respect to sending Western forces, such as military instructors, to Ukraine. France has already given SCALP cruise missiles that could strike Russia from Ukraine and was expected to send more, although that plan may now fall through.

Despite Mr. Bardella’s assurances, the National Rally is at heart nationalistic and drawn to autocratic leaders like President Vladimir V. Putin of Russia, whose annexation of Crimea in 2014 it supported. It is intent on returning power from E.U. headquarters in Brussels to national capitals, xenophobic in its belief that immigrants dilute some essential Frenchness, and determined to reintroduce some border controls in the Schengen area of 29 European countries that have officially abolished borders between them.

The party stands, in short, at a great remove from everything Mr. Macron has devoted his political life to advancing and the United States has spent untold treasure over the postwar years supporting: a Europe moving toward ever closer union and away from nationalism in the cause of peace.

“We must be lucid about the fact that our Europe is mortal,” Mr. Macron declared in April before an audience of government ministers, European ambassadors and other dignitaries at the Sorbonne. “It can die. It can die and whether it does depends entirely on our choices.”

He spoke for almost two hours but political energy had already passed to a different vision — of the nation resurgent — and Mr. Macron was not ready to see that.

For Mr. Trump, unlike Mr. Biden, NATO and the European Union hold no particular value. During his campaign this year, he said he would encourage Russia to do “whatever the hell they want” to any NATO member country that does not meet spending guidelines on defense. A growing number of NATO countries have now met the target of spending 2 percent of total output, but Mr. Trump’s America-first hostility to the alliance endures.

NATO and the European Union have been the building blocks of the postwar interlocking system built by the United States and Europe to spread prosperity and peace. They are resilient institutions but, between the war in Ukraine and a rising tide of nationalism, they have seldom faced such daunting challenges.