Frank Gaffney, head of the Center for Security Policy in Washington, has been criticized as a conspiracy theorist who regularly espouses harmful anti-Islam and anti-Muslim views. But that hasn’t stopped a few Republican presidential candidates from agreeing to participate in an event hosted Monday by Gaffney.

Ahead of Tuesday’s GOP debate in Las Vegas, the Center For Security Policy was expected to host candidates Ben Carson and Rick Santorum for a Nevada summit on the Islamic terror threat and illegal immigration, according to a report by Media Matters. Republican candidate Carly Fiorina was not slated to attend the summit but sent a video message to be played during a forum, her campaign told the Huffington Post. Billionaire real estate mogul Donald Trump also received an invite to the summit.

The summit includes panels on “border insecurity and illegal immigration” and "the threat from Iran, Sharia [law] and the global jihad movement," Media Matters reported. Gaffney, a radio host and columnist for the right-leaning Washington Times newspaper, recently gained national attention after GOP presidential front-runner Donald Trump cited Gaffney’s debunked study indicating that more than half of U.S. Muslims believe they should have the option of being governed by Shariah law, the basic Islamic legal system derived from teachings in the Quran. Trump cited Gaffney’s study as justification for his proposal to completely ban Muslim immigration and travel to the U.S.

The Southern Poverty Law Center, an anti-hate civil rights group based in Alabama, has labeled Gaffney one of "America's most notorious Islamophobes," who has "paranoid fantasies about Muslims." But despite his reputation, numerous Republican leaders in Congress have appeared on Gaffney’s radio program. Presidential hopefuls Sen. Ted Cruz of Texas, former New York Gov. George Pataki, former Arkansas Gov. Mike Huckabee and Trump have previously addressed Gaffney events in person or by video message, according to media reports.