On May 18, President George W. Bush stood before the annual conference of the American Israel Public Affairs Committee in Washington and spoke effusively to its members.

"AIPAC is doing important work," Bush said. "In Washington and beyond, AIPAC is calling attention to the great security challenges of our time.

"You've always understood and warned against the evil ambition of terrorism and their networks," the president continued. "In a dangerous new century, your work is more vital than ever."

Just over three months later, the most powerful pro-Israel lobbying organization in the United States is embroiled in an FBI probe into whether Pentagon officials gave AIPAC representatives classified material -- which sources said may have included information about Iran -- and whether they in turn passed it to the Israeli government.

For AIPAC, the allegations are potentially devastating to its credibility and large influence in Washington, and its officials have vigorously denied any wrongdoing.

"What really is troubling is the issue of dual loyalties," said Rep. Robert T. Matsui (D-Calif.), one of many senior members of the House who have defended AIPAC. The leaked allegations "raise a troubling specter" with the questioning of AIPAC's loyalty that Matsui said he is acutely sensitive to as a Japanese American who was interned in 1940 at age six months with his U.S.-born parents.

AIPAC executive director Howard Kohr said, "The feelings for Israel [in America] are quite deep. It is a country that shares our interests, our values and has been a valuable democratic ally. . . . since the founding of Israel. This is not something that is going to change. . . . I think AIPAC is going to be stronger at the end of this."

When the existence of the investigation of AIPAC became public more than a week ago, Pentagon officials said the inquiry was focused on one mid-level Pentagon analyst -- Lawrence A. Franklin -- and whether he gave a draft presidential directive on Iran to AIPAC.

Last week, however, The Washington Post reported that the two-year-old probe was broader and that sources said investigators were probing whether other defense officials had given sensitive materials to AIPAC and Ahmed Chalabi, the former Iraqi dissident who was a favorite at the Pentagon before the war. The material in question, sources said, included information not just on Iran but also on the Middle East peace process and on U.S. intentions in Iraq.

If reports of a probe involving AIPAC are true, the group said in a statement, "then surely the confidence demonstrated by the President of the United States, the highest officials in the executive branch [and] key members of the intelligence committees in continuing to regularly meet and address AIPAC during this period of time is substantial vindication of AIPAC's loyalty and trustworthiness."

The allegations have, nonetheless, profoundly shaken members of AIPAC, known as one of the capital's toughest and most effective behind-the-scenes lobbies.

Much of AIPAC's influence lies in the extensive civic participation of its supporters, the high-profile role of board members in making and raising campaign contributions, the strong ties of AIPAC to the government of Israel, and the strategic importance of Israel to U.S. interests. Every two years, AIPAC offers each new member of Congress a trip to Israel for a week to 10 days.

Israel was established in 1948, and AIPAC was set up six years later. It now has 85,000 members, an annual budget of $33.4 million and a staff of 165, with offices in Washington, 10 states and Israel.

In 2003, the organization reported spending $1.28 million on lobbying. Though not insubstantial by Washington standards, it is a fraction of the amounts spent by one of the largest lobbying groups, the AARP, which dedicated $20.9 million to lobbying last year.

The 50 members of the board of directors are a cross section of influential figures in Republican and Democratic politics and in civic affairs across the country. They include Steven Grossman, former chairman of the Democratic National Committee, and former senator Rudy Boschwitz (R-Minn.), a major Bush fundraiser. Norman Brownstein, a prominent Denver lawyer, was an early financial backer of the Kerry campaign.

Since 2000, the board members have contributed an average of $72,000 each to campaigns and political committees. One in every five AIPAC board members is a top fundraiser for Kerry or Bush.

The federal investigation itself has produced the most recent demonstration of AIPAC's power and standing, in the outpouring of support for the organization from U.S. officials that began hours after news of the federal inquiry broke.

"I know AIPAC; I know the AIPAC leadership. It is an outstanding organization," said Senate Majority Leader Bill Frist (R-Tenn.). Senate Minority Leader Thomas A. Daschle (D-S.D.) declared, "America is better and stronger for" AIPAC.

Two days after the first news reports, Republican politicians -- normally wary of controversy -- turned out in force at an AIPAC-sponsored event outside the GOP convention in New York. By AIPAC's count, the attendees included more than 60 House members, eight senators, five governors, two Bush Cabinet members and Bush campaign manager Ken Mehlman.

Through more than 2,000 meetings with members of Congress, AIPAC activists help pass more than 100 pro-Israel legislative initiatives a year. On its Web site, AIPAC lists priorities including legislation to curb Iran's nuclear program; procuring nearly $3 billion in aid for Israel; and funding U.S.-Israeli efforts to build a defense against unconventional weapons.

AIPAC does not have a political action committee and does not endorse candidates. But it is widely viewed by friends and foes as wielding significant political power.

In 2002, two Democrats in Congress with records of voting against Israel's interests -- Reps. Earl Hilliard of Alabama and Cynthia McKinney of Georgia -- faced primary opponents who received substantial support from Jewish donors. A majority of AIPAC board members gave to either McKinney's challenger or Hilliard's or both. Hilliard and McKinney lost. Bill Banks, McKinney's campaign manger, charged that AIPAC had made her the "No. 1 candidate to try to remove from office." AIPAC denied the accusation.

Even some Israeli officials have said they are uncomfortable with AIPAC's influence in U.S. politics.

"They have the threat of voting out [congressional] representatives," said Yossi Beilin, a Cabinet minister under former Labor Party prime minister Ehud Barak. "I never liked this leverage they have. It's counterproductive. Some see AIPAC as the long arm of Israel, even if it's totally wrong."

Over the years, AIPAC has accumulated many critics, including angry competitors, who feel their voices on Middle East policy have been subordinated to the policy positions taken by AIPAC.

Rep. David R. Obey (Wis.), the ranking Democrat on the House Appropriations Committee and a supporter of the policies of the more dovish Labor Party in Israel, contends that AIPAC has "pushed the Likud Party line and in the process has crowded out other voices in the Jewish community," especially those pressing for withdrawal from West Bank settlements as a concession in the peace process.

Some Israeli politicians also accuse AIPAC of representing the more hawkish factions within the Israeli government.

"They had their own agenda," said Beilin, a longtime participant in Israeli-Palestinian peace efforts. "They contradicted our government. When there was a unity government, they would say, 'But you only represent Labor,' even if I was representing the prime minister."

While many Israeli and AIPAC supporters have said it is unnecessary for Israel to engage in espionage inside the United States because of the close relationship between the Bush and Sharon administrations, some officials and observers argue that Israel has a strong interest in collecting information that could help it influence U.S. policy on Iran, which Israeli officials say is their country's greatest enemy. Both the United States and Israel have advocated strong stands against Iran's development of nuclear weapons, but Israel has urged more aggressive action than Washington. The Pentagon's Franklin, who officials said is cooperating with authorities, is suspected of sharing a document concerning the debate within the Bush administration over policy on Iran.

Ifraim Sneh, a Labor Party member of the Israeli parliament and former Cabinet member, said he has met with AIPAC members in informational exchanges for years.

"When I tell AIPAC people what the situation is here, and they tell me what the situation is in Washington, I think it's legitimate," Sneh said. "There's nothing wrong with it. It's normal. Washington is a town where the number one commodity is political gossip and political information."

Researcher Lucy Shackelford contributed to this report. Moore reported from Jerusalem.