JERUSALEM >> Ezra Nawi, an Israeli Jewish plumber, has a long history as a left-wing activist helping Palestinians in their struggle against Israel’s occupation of the West Bank. Now he is under arrest in Israel, after a right-wing activist surreptitiously filmed him bragging about ratting out Arab brokers who tried to sell Palestinian land to Jewish settlers. Such sales are a capital crime under Palestinian law.

Considered variously as a big-mouthed provocateur and a colorful human-rights adventurer, Nawi has become the latest symbol in the battle between advocacy groups on opposite sides of Israel’s political spectrum, and the increasingly fierce debate here over the nature of Israeli society and democracy.

The debate has heated up as Israel’s conservative government is pushing forward contentious legislation that would require nongovernmental organizations to disclose funding they receive from foreign governments in their publications, advertising and meetings with public officials. The proposed bill, which supporters say is meant to increase transparency, would apply mainly to leftist groups critical of Israel’s policy toward the Palestinians, since rightist groups mostly receive private funding from abroad, and it has already drawn harsh criticism from the Obama administration and European diplomats.

Daniel B. Shapiro, the U.S. ambassador to Israel, took the unusual step of meeting with Israel’s justice minister, Ayelet Shaked of the rightist Jewish Home Party, and then released a pointed statement afterward noting Washington’s “concern” about the bill.

“A free and functioning civil society is an essential element of a healthy democracy,” the statement said. “Governments must protect free expression and peaceful dissent and create an atmosphere where all voices can be heard.”

Prime Minister Benjamin Netanyahu of Israel later endorsed the bill, set to be introduced in Parliament as early as next week, saying, “I do not understand how a requirement for transparency is anti-democratic; the opposite is true.” He added, “Financing by governments is certainly something the public should know about.”

Netanyahu said the legislation should “require reports about the first shekel or dollar from foreign governments,” rather than apply only to groups that raise more than half their money abroad. But he also urged dropping a provision that would require representatives of foreign-funded groups who appear in parliament to wear tags saying so.

Some have compared the legislation to a 2012 law in Russia that required nongovernmental organizations to register as “foreign agents” if they raised money abroad. In India, foreign-backed groups are prohibited from engaging in political activity, a law that activists there feared was a way of curbing criticism of government policies.

Here in Israel, the legislation is part of a toxic tug of war over the boundaries of political discourse amid mounting international criticism of Israel’s policies toward the Palestinians. The dwindling left is frequently vilified as traitorous, as empowered right-wingers create ever-narrower definitions of Zionism. And the tactics are getting uglier.

Nawi, a gay Arabic speaker in his 60s and a prominent member of an Israeli-Palestinian rights group called Taayush, was not caught in a sting by the security apparatus for either Israel or the Palestinian Authority. Instead, he was tripped up by a plant from a right-wing organization, Ad Kan, which says it aims to “expose the true face” of what it terms anti-Israeli organizations.

Video from Ad Kan’s hidden camera was broadcast by the respected television documentary program “Uvda” — like an Israeli “60 Minutes” — on Jan. 7. A court-imposed gag order on Nawi’s arrest — for, among other charges, contact with a foreign agent and conspiracy to commit a crime — was lifted Thursday.

It is an odd case. Nawi, described in a 2009 New York Times profile as “the Robin Hood of the South Hebron Hills,” helping Palestinians who love him and “thwarting settlers and soldiers who view him with contempt,” is now accused of endangering the lives of Palestinians. That is because selling land to Israeli Jews is punishable by death according to the Palestinian Authority. Although the authority is not known to have carried out any executions for any offense in more than a decade, there have been reports of torture in its prisons.

The Ad Kan video, from about a year ago, shows Nawi behind the wheel of his jeep, bragging about what appeared to be a dubious sideline to his activist work in the West Bank. He told the man sitting next to him, whom he believed to be a fellow sympathizer, that he sometimes posed as a land broker and engaged with other land dealers mediating sales of Palestinian-owned land to Jewish settlers, then handed over their details to the Palestinian Authority security services.

Asked what the Authority did with such people, Nawi said it “catches them and kills them.”

Days later, he was arrested at the airport as he was about to leave the country.

The Ad Kan sting also embroiled Nasser Nawajah, a Palestinian from the hills of the southern West Bank, who was filmed suggesting that Nawi arrange a meeting with a land dealer in an area under Palestinian Authority jurisdiction to facilitate the dealer’s arrest. Nawajah, a field worker for B’Tselem, an Israeli human rights group that monitors Israel’s treatment of Palestinians, and a third activist, Guy Butavia, an Israeli, were also detained in connection with the case.

B’Tselem, in a statement, said that Nawajah reported to the Palestinian Authority that a Palestinian citizen of Israel, “purporting to be a land dealer,” was trying to sell land partly owned by Nawajah’s family. “This is the only legitimate course of action for Palestinians” in such cases, the statement said, adding that the land dealer was alive and well, and had never been arrested.

An Israeli court ordered Nawajah’s release Thursday on the grounds that Israel had no jurisdiction in the matter. But all three men are expected to remain in custody until at least Sunday.

Ad Kan activists say they have been planting their moles in leftist organizations for more than two years and promise to reveal additional material that they say will embarrass the groups. The exposure comes on the heels of another campaign by Im Tirtzu, an Israeli ultranationalist group, which labeled leaders from four leftist groups, including B’Tselem, as “foreign agents” and equated their work on behalf of Palestinians with support for terrorism.

For the left in Israel, the virulence of the rightist campaigns reflect an increasingly hostile environment in which liberal values and free speech appear to be under attack. The promotion of the Transparency Law by the justice minister, Shaked, has added to the sense of threat hovering over the organizations.

Netanyahu, though, likened the proposed legislation to a congressional requirement that witnesses at hearings disclose any federal grants or contracts of payments they receive originating with a foreign government.

A final draft of the bill has not yet been published. But Peace Now, another Israeli group that opposes Israel’s occupation of the territories captured in the 1967 war, said it did not resemble the standing rules of the House of Representatives. Instead, Peace Now said, it includes “draconian clauses that consist of severe violations of freedom of speech which do not exist in any other democratic country.”

The split over the bill reflects the broader political divide. Gerald Steinberg, a political-science professor and president of NGO Monitor, an advocacy group that tracks the work of nongovernmental organizations critical of Israel, said the amount of funding provided by governments of the European Union was “out of proportion.” Describing it as “a challenge to Israeli sovereignty,” he said it gave “a very narrow part of Israeli civil society a major boost in terms of their ability to influence internal debate and the marketplace of ideas.”

But Talia Sasson, a lawyer who worked in the state attorney’s office and is now the president of the board of the New Israel Fund, a nonprofit group that promotes civil rights in Israel and is a frequent target of criticism from the right, said the connotation of the transparency bill was “to tell the Israeli public that these organizations work against the state.”