Since the days when she was first a candidate for Senate, and continuing through her current campaign for President, Hillary Clinton has been unwaveringly, unapologetically hawkish on the issue of Israeli security. And for that entire time, she has enjoyed the enthusiastic and financially valuable support of Hani Masri.

Mr. Masri, 65, is a Palestinian-American businessman who is one of the most influential supporters of the Palestinian cause in Washington. The Clinton campaign recently gave Mr. Masri the title of “HillRaiser” for bundling more than $100,000 for her in the race’s first fund-raising quarter.

The obvious question is what to make of the relationship between one of Washington’s leading advocates for Palestinian issues and Mrs. Clinton, who comes in for regular praise from the pro-Israel lobbying group AIPAC.

Part of the explanation, according to some of Mr. Masri’s colleagues, is simply that Mr. Masri is a pragmatist.

“Whether a person agrees with a point of view or not, if they are a person who is going to potentially be in power, then you are obligated to deal with them, because who else are you going to deal with?” said Rafi Dajani, the executive director of the American Task Force on Palestine, a nonprofit organization that counts Mr. Masri as a member and advocates American backing of a peaceful, independent Palestinian state.

Certainly, Mr. Masri is practiced at operating among people in high places.

With long silver hair, brown eyes under bushy eyebrows and a taste for good food and wine, he has the ear of some of the most powerful Democrats in the country, including Bill Clinton and of former Democratic National Committee chairman Terry McAuliffe.

Outside of American politics, he has been known to mix with the likes of former Israeli Prime Minister Shimon Peres, supermodel Naomi Campbell, former World Bank president James Wolfensohn and Quincy Jones.

In addition to being something of an all-star mingler, though, Mr. Masri is a facilitator. He can frequently be found anchoring get-togethers of Beltway personalities from a popular corner table at the Café Milano.

“To put it mildly, he is an extraordinarily generous and gracious host,” said Hassan Nemazee, another of Mrs. Clinton’s top bundlers, who knows Mr. Masri socially. “I have been at events where he has been nice enough to invite quite a number of people to come to the Café Milano as his guests. The food is great, the wine is delicious, and he is quite the raconteur. At the end, he always has an excellent cigar in his pocket to share with you.”

Mr. Masri’s involvement with Mrs. Clinton’s career has been popping up as a story ever since she was first a candidate for office.

Back in 2000, an article in the New York Post called attention, disapprovingly, to Hillary Clinton’s raising of “$50,000 at the Washington home of Hani Masri, a Yasser Arafat intimate.” That same year, The Forward asked what Mr. Masri’s support augured for “the stance she would take on Middle East issues if elected to the Senate.”

The answer to that last question is that he has had little discernable impact, at least publicly. Mrs. Clinton, who backs the idea of a two-state solution, has been unwavering in her support of Israel since becoming Senator.

But then, say Mr. Masri’s colleagues, his involvement has never been about public gestures. Whatever lobbying he has done has been of the low-key variety, at his events at the Café Milano and elsewhere.

“A lot of his advocating for Palestinian issues is done in these types of lunches,” said Mr. Dajani. “He has access to people who have influence on decision-making, and that’s a relatively small circle in Washington D.C. And I think he has put it to good use in terms of arguing for a two-state solution.”

Mr. Masri has become one of the party’s top donors thanks to the vast wealth he has accumulated through his Capital Investment Management Corporation, which is based in Washington, where he lives with his wife in a mansion in the northwest quarter. The political donations he has made, to the D.N.C. and other Democratic candidates—including Mrs. Clinton’s leading Democratic rival, Barack Obama, when he ran for the Senate—certainly suggest that his support is a sought-after commodity.

“Mr. Masri has been a longtime supporter of Senator Clinton, her husband and the Democratic Party, and we are grateful for that support,” said Mrs. Clinton’s spokesman, Howard Wolfson.

Certainly, that support has not been conditioned on Mrs. Clinton’s views on the Middle East. In her seven years as a Senator, Mrs. Clinton has shown scant evidence of her once-unusual level of support of the Palestinian cause. (She came out in support of an independent Palestinian state when she was First Lady, long before that became a mainstream position for American politicians.)

Since moving to New York, Mrs. Clinton has backed the building of the controversial wall separating Israel from the West Bank and has called a nuclear-armed Iran an “unacceptable” threat to Israeli safety.

Last July, during the Israeli bombing of Lebanon, Mrs. Clinton told a pro-Israel rally held in midtown that she supported “whatever steps are necessary” to defend Israel in response to what she called the “unwarranted, unprovoked” seizure of three Israeli soldiers by members of Hamas and Hezbollah, “the new totalitarians of the 21st century.”

Last month, she co-sponsored a resolution calling for the immediate and unconditional release of Israeli soldiers held captive by Hamas and Hezbollah. And she excoriated as indoctrination new Palestinian schoolbooks that described Israel’s founding as “a disaster unprecedented in history.”

Mr. Masri, who once hosted Arafat during a visit to Washington, according to a report in The Forward, may at least concur with Mrs. Clinton’s criticism of Hamas, which is currently battling its governing partner, Fatah, in fierce firefights on the streets of Gaza.

Some veterans of the Middle East peace effort stressed that ultimately, Mr. Masri and Mrs. Clinton want the same thing: more rigorous United States involvement to help negotiate a peaceful two-state resolution in the Middle East.

“Hani is totally a man of peace,” said Uri Savir, who was Israel’s chief negotiator during the Oslo Accords and a close ally of Mr. Peres. “He was very close to Bill Clinton—I think Clinton did seek his advice in the past on Palestinian politics.”

Mr. Savir worked closely with Mr. Masri to invest $60 million of Israeli and Palestinian money in a fund organized by the Peres Peace Center to help promote peace through business relationships, a project that he says was destroyed by the coming of the intifada. They have worked closely together ever since.

Mr. Savir stressed that Mr. Masri’s main concern was American interests. He used his relationship with Arafat to try and persuade the late P.L.O. leader “to go for the Clinton plan,” Mr. Savir said, and since the years when Arafat would seek him out as a United States contact, he has largely stayed out of Palestinian domestic politics.

Nabil Abuznaid, deputy chief of the P.L.O. mission in Washington, said that Mr. Masri was “close to Arafat and his government and not connected to the religious groups” and is “considered in the peace camp.” He also said that Mr. Masri has little if any influence within the Palestinian Authority under President Mahmoud Abbas.

In an e-mail exchange with The Observer, Mr. Masri described his own role as an unofficial facilitator of negotiation with Israel.

“I was never an official with Yasir Arafat or the Palestinian Authority,” he wrote. “I tried to help with the peace process between the Palestinians and Israelis and tried to help them approve the Clinton peace initiative.”

As for his latest mission—to get another Clinton elected to the White House—Mr. Masri explained it this way:

“Senator Clinton and I don’t always agree. But I know that the nation would be well served with someone with her experience and depth of knowledge.”