As a child growing up a Maronite Christian in war-torn southern Lebanon in the 1970s, Ms. Gabriel said, she had been left lying injured in rubble after Muslims mercilessly bombed her village. She found refuge in Israel and then moved to the United States, only to find that the Islamic radicals who had terrorized her in Lebanon, she said, were now bent on taking over America.

“America has been infiltrated on all levels by radicals who wish to harm America,” she said. “They have infiltrated us at the C.I.A., at the F.B.I., at the Pentagon, at the State Department. They are being radicalized in radical mosques in our cities and communities within the United States.”

Through her books, media appearances and speeches, and her organization, ACT! for America, Ms. Gabriel has become one of the most visible personalities on a circuit of self-appointed terrorism detectors who warn that Muslims pose an enormous danger within United States borders.

Representative Peter T. King, Republican of Long Island, will conduct hearings Thursday in Washington on a similar theme: that the United States is infiltrated by Muslim radicals. Mr. King was the first guest last month on a new cable television show that Ms. Gabriel co-hosts with Guy Rodgers, the executive director of ACT! and a Republican consultant who helped build the Christian Coalition, once the most potent political organization on the Christian right.

Ms. Gabriel, 46, who uses a pseudonym, casts her organization as a nonpartisan, nonreligious national security group. Yet the organization draws on three rather religious and partisan streams in American politics: evangelical Christian conservatives, hard-line defenders of Israel (both Jews and Christians) and Tea Party Republicans.

She presents a portrait of Islam so thoroughly bent on destruction and domination that it is unrecognizable to those who study or practice the religion. She has found a receptive audience among Americans who are legitimately worried about the spread of terrorism.

But some of those who work in counterterrorism say that speakers like Ms. Gabriel are spreading distortion and fear, and are doing the country a disservice by failing to make distinctions between Muslims who are potentially dangerous and those who are not.

Brian Fishman, a research fellow at both the New America Foundation in Washington, and the Combating Terrorism Center at the United States Military Academy at West Point, said, “When you’ve got folks who are looking for the worst in Islam and are promoting that as the entire religion of 1.5 or 1.6 billion people, then you only empower the real extremists.”

Ms. Gabriel is only one voice in a growing circuit that includes counter-Islam speakers likePamela Geller, Robert Spencer and Walid Shoebat. What distinguishes Ms. Gabriel from her counterparts is that she has built a national grass-roots organization in the last three years that has already engaged in dozens of battles over the place of Islam in the United States. ACT! for America claims 155,000 members in 500 chapters across the country. To build her organization, Ms. Gabriel has enlisted Mr. Rodgers, who had worked behind the scenes for the Christian Coalition’s leaders, Ralph Reed and the television evangelist Pat Robertson. (Ms. Gabriel herself was once an anchor for Mr. Robertson’s Christian television network in the Middle East).

As national field director, Mr. Rodgers planted and tended Christian Coalition chapters across the country, and is now using some of the same strategies as executive director of ACT! Among those tactics is creating “nonpartisan voter guides” that rank candidates’ responses and votes on issues important to the group.

Just as with the Christian Coalition’s voter guides, the candidates whose positions most often align with ACT!’s are usually Republicans. Mr. Rodgers previously served as campaign manager for Patrick J. Buchanan’s presidential run in 1996, and as a consultant for John McCain in 2008.

Ms. Gabriel and Mr. Rodgers declined to be interviewed in person or over the telephone, but agreed to respond to questions by e-mail. They permitted interviews with only their national field director and two chapter leaders they selected, though half a dozen other interviews were conducted with chapter leaders before they were told not to talk.

Ms. Gabriel says she is motivated not by fear or hatred of Islam, but by her love for her adopted country.

“I lost Lebanon, my country of birth, to radical Islam,” she wrote. “I do not want to lose my adopted country America.”

She insists that she is singling out only “radical Islam” or Muslim “extremists” — not the vast majority of Muslims or their faith. And yet, in her speeches and her two books, she leaves the opposite impression. She puts it most simply in the 2008 introduction to her first book, “Because They Hate: A Survivor of Islamic Terror Warns America.”

“In the Muslim world, extreme is mainstream,” she wrote. She said that there is a “cancer” infecting the world, and said: “The cancer is called Islamofacism. This ideology is coming out of one source: The Koran.”

In what ACT! is calling “Open a Koran” day this September, the group plans to put up 750 tables in front of post offices, libraries, churches and synagogues and hand out leaflets selectively highlighting verses that appear to advocate violence, slavery and subjugation of women.