With violence across the Middle East fixing Islam smack at the center of the American political debate, an organization partly financed by donors closely identified with wealthy Persian Gulf governments has emerged as the most vocal advocate for American Muslims — and an object of wide suspicion. The group, the Council on American-Islamic Relations, defines its mission as spreading the understanding of Islam and protecting civil liberties. Its officers appear frequently on television and are often quoted in newspapers, and its director has met with President Bush. Some 500,000 people receive the group's daily e-mail newsletter. Yet a debate rages behind the scenes in Washington about the group, commonly known as CAIR, its financing and its motives. A small band of critics have made a determined but unsuccessful effort to link it to Hamas and Hezbollah, which have been designated as terrorist organizations by the State Department, and have gone so far as calling the group an American front for the two. In the latest confrontation yesterday, CAIR held a panel discussion on Islam and the West in a Capitol meeting room despite demands by House Republicans that Speaker Nancy Pelosi, a Democrat, not allow the event. The Republicans called its members "terrorist apologists." Caley Gray, a spokesman for Representative Bill Pascrell Jr., a New Jersey Democrat who helped book the room, rejected that label in a phone interview and said CAIR held similar meetings when Congress was controlled by Republicans. Still, Mr. Gray called back to specify that Mr. Pascrell did not endorse all of the group's positions. Last fall, Senator Barbara Boxer of California issued a routine Certificate of Appreciation to the organization representative in Sacramento, but she quickly revoked it when critics assailed her on the Web under headlines like "Senators for Terror." "There are things there I don't want to be associated with," Ms. Boxer said later of the revocation, explaining that her California office had not vetted the group sufficiently. CAIR and its supporters say its accusers are a small band of people who hate Muslims and deal in half-truths. Ms. Boxer's decision to revoke the Sacramento commendation provoked an outcry from organizations that vouch for the group's advocacy, including the American Civil Liberties Union and the California Council of Churches. "They have been a leading organization that has advocated for civil rights and civil liberties in the face of fear and intolerance, in the face of religious and ethnic profiling," said Maya Harris, the executive director of the A.C.L.U. of Northern California. Government officials in Washington said they were not aware of any criminal investigation of the group. More than one described the standards used by critics to link CAIR to terrorism as akin to McCarthyism, essentially guilt by association. "Of all the groups, there is probably more suspicion about CAIR, but when you ask people for cold hard facts, you get blank stares," said Michael Rolince, a retired F.B.I. official who directed counterterrorism in the Washington field office from 2002 to 2005. Outreach to all Muslims via groups they support is an important aspect of ensuring that extremists cannot get a foothold here as they have in Europe, Mr. Rolince said. The cloud kicked up by the constant scrutiny is such that spokesmen at several federal agencies refused to comment about the group and some spoke only on the condition of anonymity.