The brutal shedding of Jewish blood on Oct. 7, followed by Israel’s relentless military assault on Gaza, has brought a fraught question to the fore in a moment of surging bigotry and domestic political gamesmanship: Is anti-Zionism by definition antisemitism?

The question deeply divided congressional Democrats last week when Republican leaders, seeking to drive a wedge between American Jews and the political party that three-quarters of them call their own, put it to a vote in the House. It has shaken the country’s campuses and reverberated in its city streets, where pro-Palestinian protesters bellow chants calling for Palestine to be free from the Jordan River to the Mediterranean Sea.

It surfaced in Wednesday’s Republican presidential debate, when Nikki Haley, the former South Carolina governor, said, “If you don’t think Israel has a right to exist, that is antisemitic.” The following night, lighting the national menorah behind the White House, Vice President Kamala Harris’s husband, Doug Emhoff, who is Jewish, warned, “When Jews are targeted because of their beliefs or identity, and when Israel is singled out because of anti-Jewish hatred, that is antisemitism.”

Zionism as a concept was once clearly understood: the belief that Jews, who have endured persecution for millenniums, needed refuge and self-determination in the land of their ancestors. The word still evokes joyful pride among many Jews in the state of Israel, which was established 75 years ago and repeatedly defended itself against attacks from Arab neighbors that aimed to annihilate it.

If anti-Zionism a century ago meant opposing the international effort to set up a Jewish state in what was then a British-controlled territory called Palestine, it now suggests the elimination of Israel as the sovereign homeland of the Jews. That, many Jews in Israel and the diaspora say, is indistinguishable from hatred of Jews generally, or antisemitism.

Yet some critics of Israel say they equate Zionism with a continuing project of expanding the Jewish state. That effort animates an Israeli government bent on settling ever more parts of the West Bank that some Israelis, as well as the United States and other Western powers, had proposed as a separate state for the Palestinian people. Expanding those settlements, to Israel’s critics, conjures images of “settler colonialists” and apartheid-style oppressors.

So for some Jews, the answer to the question is obvious. Of course anti-Zionism is antisemitism, they say: Around half the world’s Jews live in Israel, and destroying it, or ending its status as a refuge where they are assured of governing themselves, would imperil a people who have faced annihilation time and again.

“There is no debate,” said Jonathan Greenblatt, the chief executive of the Anti-Defamation League, which has been defining and monitoring antisemitism since 1913. “Anti-Zionism is predicated on one concept, the denial of rights to one people.”

Many Palestinians and their allies recoil just as fiercely: The equating of opposition to a Jewish state on once-Arab land — or opposition to its expansion — with bigotry is to silence their national aspirations, muffle political dissent and denigrate 75 years of their suffering.

Laila el-Haddad, a Palestinian activist and author, called it “a chilling attempt to punish and silence voices critical of Israeli policies.”

But perhaps nowhere is the question more fraught than among Jews themselves. Younger, left-leaning Jews, steeped in the cause of antiracism and terms like “settler colonialism,” are increasingly searching for a Jewish identity centered more on religious values like the pursuit of justice and repairing the world than on collective nationalism tied to the land of Israel.

Many older liberal Jews have also struggled with the Israeli government’s lurch to the far right, but they see Israel as the centerpiece and guarantor of continued Jewish existence in an ever more secular world.

“We’re living in an increasingly post-religious age, and any Jewish community that walks away from the Jewish people, and its most articulate expression of our times — the Jewish state, the state of Israel — is walking away from their own future,” said Ammiel Hirsch, the senior rabbi of Stephen Wise Free Synagogue in Manhattan and the founder of Amplify Israel, which seeks to emphasize the Jewish state in Jewish worship.

For Republicans, the issue is simple and convenient. The raising of anti-Zionism in the debate over antisemitism amid the Israel-Hamas war pushes aside the presence of white-nationalist bigots on the fringes of the Republican coalition — like Nick Fuentes, the avowed neo-Nazi who dined with Kanye West and former President Donald J. Trump last year — and instead forces Democrats to defend the pro-Hamas demonstrators on their own coalition’s fringes.

So on Tuesday, when G.O.P. leaders led by Representative David Kustoff of Tennessee, one of the House’s two Jewish Republicans, put to a vote a resolution condemning all forms of antisemitism and flatly stated “that anti-Zionism is antisemitism,” the 216 Republicans who voted yes included two who have been accused of antisemitism and white-nationalist flirtations, Representatives Paul Gosar of Arizona and Marjorie Taylor Greene of Georgia. (The one Republican who voted no, Representative Thomas Massie of Kentucky, has now been labeled antisemitic by the White House.)

For the broader Democratic community, by contrast, the debate has been wrenching, pitting allies against one another, splintering more conservative Jewish Democrats who absolutely believe anti-Zionism is antisemitic from progressive Democrats, especially Democrats of color, who argue just as strongly for the latitude to criticize Israel, and leaving a huge middle unwilling to draw bright lines.

Thirteen Democrats voted no, including Israel’s fiercest critics in Congress, Ilhan Omar of Minnesota, Rashida Tlaib of Michigan and Alexandria Ocasio-Cortez of New York. Ninety-five voted yes, but 92 Democrats voted “present,” among them prominent Jews like Jerrold Nadler of New York, Jamie Raskin of Maryland and Jan Schakowsky of Illinois.

“Folks, this isn’t complicated: MOST antizionism — the type that calls for Israel’s destruction, denying its right to exist — is antisemitic. This type is used to cloak hatred of Jews,” Mr. Nadler wrote on social media after the vote. “Some antizionism isn’t that. Thus, it’s simply inaccurate to call ALL antizionism antisemitic.”

In fact, it is complicated. Jonathan Jacoby, the director of the Nexus Task Force, a group of academics and Jewish activists affiliated with the Bard Center for the Study of Hate, said the group had wrestled with the issue for several years now, seeking a definition of antisemitism that captures when anti-Zionism crosses from political belief to bigotry. He warned that shouting down any political action directed against Israel as antisemitic made it harder for Jews to call out actual antisemitism, while stifling honest conversation about Israel’s government and U.S. policy toward it.

The definition of antisemitism as drafted by the International Holocaust Remembrance Alliance and embraced by the Trump White House includes phrases that critics say squelch political — not hate — speech:

• Denying the Jewish people their right to self-determination, such as by claiming that the existence of a State of Israel is a racist endeavor.
• Applying double standards by requiring of Israel behavior not expected or demanded of any other democratic nation.
• Comparing contemporary Israeli policy to that of the Nazis.

The Nexus definition agrees that holding Jews around the world responsible for Israeli government actions, as pro-Palestinian protesters did last week outside an Israeli restaurant in Philadelphia, is Jew hatred. It also holds that it is antisemitic to reject the right of Jews alone to define themselves as a people and exercise self-determination, as some on the left do in arguing that Jews are a religion, not a nation.

But Nexus pushes back sharply on some aspects of the I.H.R.A. definition, stating, “Paying disproportionate attention to Israel and treating Israel differently than other countries is not prima facie proof of antisemitism” and “Opposition to Zionism and/or Israel does not necessarily reflect specific anti-Jewish animus.”

Yehuda Kurtzer, the president of the Shalom Hartman Institute, a Jewish research organization, said that Judaism had always contained elements of religion and nationhood, and that Jewish identity had toggled between the two over the millenniums. It is unsurprising that the two strains can seem baffling, he said.

Since the rise of violent white supremacy that accompanied the political movement of Mr. Trump, Jewish intellectuals have viewed right-wing antisemitism “as dangerous to Jewish bodies,” Mr. Kurtzer continued. The 2018 Pittsburgh synagogue massacre that took 11 Jewish lives was perpetrated by an adherent to the “great replacement” theory, a conspiratorial fiction designed to create race hatred by holding that Jews are importing Black and brown people to supplant white Americans.

Amid such carnage, left-wing antisemitism is…

… indicted for charging Israeli soldiers with crimes against humanity or invoking the blood libel in maligning the Jewish state.

While center-right commentators have used the debate to curtail their political adversaries, urging Democrats to clean up their own house, Mr. Jacoby argued that a more entwined approach to fighting antisemitism on the far right and far left was needed. The treatment of the debate as a political tool, he feared, would leave Jews prey to both wings.