On an August Sunday in Amherst, Mass., a small peace vigil was underway near the town center, extending a weekly tradition dating back to the 1970s.

A half-dozen activists carried homemade signs with messages of pacifism and demilitarization: “Stop the killing.” “Healthcare not warfare.” “Pray for peace.”

But one sign sent a different kind of message. “We stand with Ukraine,” it read. For these global-peace idealists, it seemed, Ukraine’s was a war worth fighting — or at least supporting.

As the war in Ukraine drags on, it is not uncommon to hear peace activists and progressive politicians, including many who have opposed American interventions elsewhere, make an exception for Ukraine’s self-defense against Russia. Even as casualties mount among troops and civilians, global food shortages grow, talk of nuclear war hovers and President Biden outrages human rights groups by providing Ukraine with cluster munitions, only pockets of resistance to U.S. support for Ukraine exist on the American political left.

That is a departure from recent American political tradition, in which opposition to involvement in foreign wars, from Vietnam to Iraq to Afghanistan, has been strong on the left while conservatives have been more willing to support the flexing of American power abroad.

A CNN poll published this month found that a slight majority of Americans has come to believe that the United States has sent enough money to Ukraine. But that’s largely because of growing Republican opposition, as 74 percent of liberal Democrats support providing more. That dynamic has played out clearly in Congress, where a mid-July amendment sponsored by Representative Marjorie Taylor Greene, Republican of Georgia, to slash $300 million in aid to Kyiv drew 130 House Republican votes — but none from Democrats.

“It’s been tough,” conceded Medea Benjamin, a founder of the antiwar group Code Pink, which was founded in late 2002, mainly to oppose George W. Bush’s invasion of Iraq.

Ms. Benjamin, who was dragged out of a Senate hearing in March after she shouted “Be a diplomat — not a war hawk!” during testimony by Secretary of State Antony J. Blinken, said she firmly opposes Russia’s invasion of Ukraine but doubts that a prolonged fight can restore Ukraine’s borders. “So we’re going to let this war keep going, let more Ukrainians die, more chances of a wider war in Europe and more chance of a nuclear war, just so it’s back to where we were a month after the Russian invasion?” she asked.

That message may be reaching a wider audience as the 2024 presidential campaign gets underway. Robert F. Kennedy Jr., who is mounting a long-shot Democratic primary campaign against Mr. Biden, has argued that the more than $100 billion in American support is prolonging the war and has therefore been “terrible for the Ukrainian people.”

“We have neglected many, many opportunities to settle this war peacefully,” Mr. Kennedy said in June at a town hall event hosted by the NewsNation network.

For now, however, those voices are outliers, even within progressive circles.

Win Without War, a group founded in 2002 to promote “a better U.S. foreign policy that favors peace, not militarism,” as its motto goes, has said little about the conflict. The last entry on the group’s “Ukraine updates page” is more than a year old, and its president, Stephen Miles, said that “more often than not, President Biden has gotten it right” on Ukraine.

The grass-roots group Peace Action similarly has put little emphasis on Ukraine, and Ms. Benjamin said the organization was “internally split” on how to address the war. The most recent entry on the conflict under a “briefing and action steps” header on its website, also more than a year old, recommends that the public “support continued funding for Ukraine and Ukrainians, including humanitarian aid and refugee assistance and resettlement,” while also backing diplomacy to secure a full withdrawal of Russian troops in exchange for U.S. sanctions relief.

Jon Rainwater, the group’s executive director, said the peace movement “needs to step back and refine its approach” after U.S. “wars of choice in places like Iraq, Libya and Afghanistan.”

“Russia’s invasion flips the script,” he added. “On top of that, the country the U.S. is sending arms to here is engaging in actual self-defense. The peace movement needs to resist the urge to simply replay our ‘greatest hits’ about U.S. imperialism when we talk about Ukraine,” he said, adding that his group strongly supports intensive diplomacy to resolve the conflict.

Ms. Benjamin said she was unable to rally major environmental groups to call for a quick end to the fighting, despite the specter of nuclear conflict and risks to Ukraine’s Zaporizhzhia nuclear power plant near the conflict’s front lines.

Several reasons explain the relative quiet on the left. First is the obvious fact that Mr. Biden has not committed the U.S. military to the conflict, making for a cost in treasure but not American troops. And liberals, especially Democrats in Congress, are not very eager to critique a Democratic president’s top foreign policy project, especially when doing so would align them with the likes of former President Donald J. Trump and the former Fox News host Tucker Carlson, both of whom have denounced Mr. Biden’s support for Ukraine and called for an immediate end to the fighting.

Mr. Biden’s rationale for backing Ukraine also squares with progressive ideals, a point that the liberal theorist Michael Walzer, author of the seminal 1977 book “Just and Unjust Wars,” argued shortly after President Vladimir V. Putin of Russia launched a full-scale invasion in February 2022. Writing in The Wall Street Journal, Mr. Walzer said that “Russia’s invasion of Ukraine is illegal under international law, and it is unjust according to every version of just war theory.”

The view is common among defenders of Mr. Biden’s policies.

“This is not a war that America started. This is a war that Russia started against its neighbor, and the left generally supports a system of rules for the world in which might does not make right,” said Matthew Duss, a former top foreign policy aide to Senator Bernie Sanders, a Vermont independent. “Helping Ukraine defend itself against this invasion is something that upholds that principle.”

Mr. Sanders was one of the most vehement opponents of the Iraq War but has staunchly supported the defense of Ukraine, calling the Russian invasion “a blatant violation of international law and of basic human decency.”

Mr. Duss noted that Mr. Biden has taken steps to limit the conflict, like ruling out the deployment of American troops and the imposition of a “no fly” zone over Ukraine, drawing criticism from some conservatives who believe he has been too slow to approve weapons desperately sought by Kyiv.

Mr. Miles of Win Without War said it was only natural for his group to embrace Ukraine’s defense.

“We were founded in opposition to the war in Iraq,” he said. “For us in particular, but for progressives more broadly, we recognized very early on the historical echo of what we were seeing with Putin’s aggression in Ukraine and George W. Bush’s invasion of Iraq.”

“These were wars of choice that shouldn’t ever have been fought, and ultimately the onus for ending the war is with the aggressor,” he added.

The Biden administration has countered calls for negotiations with the argument that Mr. Putin is not a serious partner for peace, and that failure to fully repel Russia from Ukraine would reward aggression and invite further conflict in Europe and beyond.

Some critics also charge that American opponents of the war are, at best, unwittingly parroting Kremlin propaganda. “We constantly have to make clear that we are anti-Russian invasion and are in no way Putin apologists,” Ms. Benjamin said.

Ms. Benjamin noted that Mark A. Milley, the chairman of the Joint Chiefs of Staff, argued internally last fall for pressing the Ukrainians to negotiate with Moscow, on the ground that they were unlikely to improve their military position significantly. Mr. Milley was overruled by other U.S. officials, including Mr. Blinken.

Some Americans are clearly listening to the dissenters: A video clip posted on Twitter last month shows Mr. Kennedy telling the Fox News host Sean Hannity that the United States has stifled peace efforts in Ukraine and actually welcomes “war with Russia.” The post has been viewed nearly six million times.

And a 2015 lecture by the University of Chicago professor John Mearsheimer titled “Why Ukraine is the West’s fault,” in which he argues that American policy toward Ukraine has needlessly provoked Russia, has been viewed on YouTube 29 million times. Mr. Mearsheimer said that most of those views had come since the invasion last year.

He expressed incredulity at the video’s popularity, saying, “What makes it all so amazing to me is that I cannot remember giving the talk.”

But YouTube clicks have yet to translate into political impact.

“Never has the need for a global peace movement — and international peace initiatives — been more apparent,” Robert Borosage, a liberal activist, wrote in The Nation, a leftist magazine with a long antiwar tradition, in April. “Yet, on the left, the most visible voices are those condemning any deviation from total support for the war.”

Ms. Benjamin lamented that congressional Democrats, even ones often reliably critical of past American interventions abroad, “have for the most part been absolutely silent.” She noted with particular disdain the way 30 progressive members of Congress wrote — and then withdrew — a letter to Mr. Biden in October urging him to negotiate with Russia a “rapid end to the conflict.”

Code Pink has tried to change the dynamic with pressure on Democrats, like a protest in February outside the Oakland offices of Representative Barbara Lee, who was the lone member of Congress to oppose the September 2001 congressional resolution authorizing the use of military force against Al Qaeda and its allies. “Money for housing, not for war!” a handful of activists chanted at an earlier protest last summer.

Activists and political analysts alike say that a failure by Ukraine to achieve a major breakthrough in its monthslong counteroffensive could open the door to more liberal pressure on Mr. Biden to initiate diplomacy to halt the fighting.

But for now, shifting Mr. Biden’s approach has been a tough slog, something apparent even after one Code Pink member stumbled into the opportunity of a lifetime one late February evening and captured it on video.

The activist happened to be dining at the Red Hen, a Washington restaurant known for its exposed brick and hearty rigatoni, when Mr. Biden arrived for a date night with his wife.

“President Biden! I hate to bother you; we need to end this war in Ukraine,” she called out from across the dining room.

Tucked into a corner table, the president did not turn his head as she continued. “We need to push through negotiations. I hate to bother you, but people are dying!” she called.

The brief encounter ended after the restaurant’s staff implored the activist to leave. And it went largely unnoticed in the news media, as it was quickly overshadowed by a far more dramatic event.

After dinner, Mr. Biden sneaked out to Andrews Air Force Base for a secret trip to Kyiv. He appeared there with Ukraine’s president, Volodymyr Zelensky, pledging America’s unwavering support — and another $500 million — for the country’s fight against Russia.