FRANKLIN LAKES, N.J. — Senator Robert Menendez spoke in a level, matter-of-fact tone, but his words were grave. Anti-Semitism is on the march around the world, he said. Negotiations with Iran, he warned, have reached “the witching hour,” with the security of both the United States and Israel at stake.

Addressing a Sunday morning crowd at the Barnert Temple, a Reform synagogue in northern New Jersey, Mr. Menendez promised that his listeners could count on him to be vigilant.

“There can be no denying the Jewish people their legitimate right to live in peace and security,” he said, vowing just days before the Passover holiday that he would stand with Israel “so long as I have a voice and a vote in the United States Senate.”

It was the closest the senator came to acknowledging his recent troubles. For weeks, Mr. Menendez, a New Jersey Democrat, has battled to keep his standing in the Senate amid an unfolding investigation into his relationship with a political benefactor, the Florida eye surgeon Salomon Melgen, and whether Mr. Menendez improperly influenced policy on his behalf.

In his moment of political peril, Mr. Menendez, who has denied any wrongdoing, has found perhaps his deepest well of support in the expansive pro-Israel community, including prominent Jewish Democrats concerned about the direction of White House negotiations with Iran.

The top Democrat on the Senate Foreign Relations Committee, Mr. Menendez has been a vocal critic of the Iran talks, writing multiple bills to toughen economic sanctions and recently introducing legislation with Senator Bob Corker, a Tennessee Republican, to require congressional review of any nuclear deal.

This year, Mr. Menendez startled Washington by harshly mocking the Obama administration’s defense of its tactics toward Iran: The White House message, he said in January, “sounds like talking points that come straight out of Tehran.”

Mr. Menendez has found an appreciative constituency in the array of advocacy groups focused on Israel’s security, many of which have been alarmed by the talks with Iran and the eruption of tensions between the Obama administration and Israel’s prime minister, Benjamin Netanyahu.

Mr. Menendez received enthusiastic applause addressing Aipac, the prominent Israel advocacy group, this month; its leaders have contributed to his legal defense fund and have privately pressed other Israel-oriented donors to do the same.

By the end of 2014, Mr. Menendez had raised more than $200,000 for his legal fund — nearly a quarter of all its receipts — from political donors who have also given to pro-Israel political action committees, according to an examination of financial documents filed by the Robert Menendez Legal Expense Trust.

The support has been rhetorical, as well as financial: Major Jewish leaders in both parties have spoken up as character witnesses for Mr. Menendez, even suggesting that the potential charges against him might be politically motivated.

“All I hear, repeatedly, is that he is being punished for his rational and strong stance on trying to get a strong deal for America and Israel, on Iran,” said Morton A. Klein, president of the Zionist Organization of America.

Mr. Klein said he was agnostic on the merits of the legal investigation, but had encouraged allies to help Mr. Menendez in any case: “I say, look, we don’t know the validity of these allegations, but this is a man who deserves support.”

Norm Coleman, a former senator from Minnesota who sits on the board of the Republican Jewish Coalition, said Mr. Menendez could count on “broad, widespread, deep support, across the political aisle, from people who care about Israel.”

That line of thinking frustrates some Jewish and pro-Israel Democrats, who say Mr. Menendez has earned their gratitude but who will not go quite as far in alleging a conspiracy against him.

What’s more, Jewish leaders said, Mr. Menendez has won over the community on issues outside Iran’s nuclear program. He has been a staunchly liberal voice on matters of social policy; as the Senate’s only Hispanic Democrat, Mr. Menendez has been a champion of immigration reform, a popular measure in the Jewish community.

Greg Rosenbaum, chairman of the National Jewish Democratic Council, a group that has been critical of Mr. Menendez’s hawkish approach to Iran, called the notion of sinister prosecutorial action “baloney.” But Mr. Rosenbaum, who is also involved in Aipac, said he expected the senator’s pro-Israel allies to remain supportive even in the event of criminal charges.

“I think we’re pretty much lock step with his positions on issues that matter to American Jewish voters,” he explained.

Speaking in Franklin Lakes over the weekend, Mr. Menendez refrained from head-on criticism of the Obama administration, but he repeatedly staked out positions well to the president’s right, and drew chuckles by alluding to tensions with the White House. “The president and I don’t always see eye to eye these days,” he said.

There was no explicit mention of the senator’s legal predicament, but one of his hosts offered a message of reassurance to the embattled lawmaker by way of an introduction. Josh Gottheimer, a Microsoft executive, told Mr. Menendez that he could count on the Jewish community to stick with him.

“No matter what, no matter when, no matter how, we will always have your back,” Mr. Gottheimer said. “Because you’ve always had ours.”