Updated

Battle lines are being drawn as Republican Rep. Peter King prepares to hold a high-profile hearing Thursday that will examine the threat posed by radical Islam in the United States.

The New York lawmaker, who is chairman of the House Homeland Security Committee, has received threats in recent days but says he "will not back down." His plans have divided lawmakers on Capitol Hill and triggered a backlash from Muslim and civil liberties groups claiming he's unfairly targeting members of one faith.

But King said Wednesday he's moving ahead with the hearing on the basis of "where the evidence is."

"That's where the danger is coming from," he told Fox News. "It's a small percentage. The overwhelming majority of Muslims are outstanding Americans. But the reality is that the threat is coming from within that community."

King said that if he thought members of the Irish, Jewish or any other ethnic community were exhibiting threatening signs he would launch an investigation into them as well. But he said Al Qaeda is actively trying to radicalize American Muslims, with some success.

"This has nothing to do with Islam as a religion," King said.

According to a report in the New York Post, one witness -- M. Zuhdi Jasser, president of the American Islamic Forum for Democracy -- plans to say imams in America promote a "lack of cooperation and lack of assimilation" which feeds radicalization. He reportedly plans to say mosques are "feeding into that ideology."

King got an endorsement Wednesday from House Majority Leader Eric Cantor, R-Va., who suggested King is on solid ground in calling the hearing.

"It's obvious where there have been problems with terrorist activity," Cantor said. "We in this country are threatened by the spread of radical Islam."

The White House on Sunday also dispatched Denis McDonough, deputy national security adviser, to deliver a speech on Islamic radicalization Sunday at a Virginia mosque. McDonough stressed that the United States does not practice "guilt by association" but also said afterward that the administration welcomes "congressional involvement."

However, other lawmakers and groups have not been so welcoming.

Senate Majority Leader Harry Reid, D-Nev., slammed King in a statement Wednesday, warning the hearing could divide Americans based on religion.

"I take the threat of terrorism very seriously, and no one is more committed to hunting down terrorists and bringing them to justice, wherever they live, than I am," Reid said. "But I am deeply concerned about these hearings, which demonize law-abiding American Muslims who make important contributions to our society, as I would be about congressional hearings to investigate Catholics, Jews or people of any other faith based solely on their religion."

The Council on American-Islamic Relations joined other groups for a press conference Wednesday denouncing the hearing. CAIR Director Nihad Awad said that King's "bias" and "fear-mongering" make him "unfit" to lead the House committee. Awad condemned violent extremism but said King was spreading "false" allegations and "irresponsible rhetoric" about American Muslims.

CAIR was among the groups that backed a protest in New York City over the weekend.

Dozens of organizations, including the American Civil Liberties Union, also sent out a letter Tuesday comparing King's plans to "McCarthyism and Japanese internment."

Rep. Keith Ellison, D-Minn., who is Muslim, criticized King in an interview Wednesday, saying he was assigning "blame" before the hearing even begins.