Should the U.S. ban Muslims from entering the country? Is radical Islam bent on taking over America? 

Ten or fifteen years ago, questions like these were largely relegated to the political fringes. Now, they help define the campaigns of the two leading Republican candidates, Donald Trump and Ted Cruz, who are competing in the New York primary on April 19.

The candidates are voicing ideas that come from a deliberate and well-funded campaign to convince the country that even regular, law-abiding Muslims represent a threat to national security.

The campaign kicked into high gear in 2010 in New York, with the opposition to Park51, also known as the 'ground zero mosque.' While some protesters saw the project as an affront to the memory of those who died on 9/11, others took it a step further, arguing that it was a 'victory mosque' meant to celebrate the attacks.

Despite the considerable misinformation spread about the project and the people behind it, the controversy helped ignite a wave of opposition to mosques across America: in places like Sheboygan, Wisconsin, Temecula, California and Murfreesboro, Tennessee. At the same time, state legislators started passing laws to ban Sharia law (usually couched as a ban on foreign law), often in states with relatively few Muslims. 

One of the principal players in this movement is Frank Gaffney, a former official in the Reagan administration who runs the Center for Security Policy, in Washington. He's now a foreign policy advisor for Ted Cruz. Gaffney's security think tank funded the website run by Pam Geller, a leader of the opposition to the ground zero mosque. He also helped stoke right-wing suspicions that President Obama is Muslim, has accused a variety of both liberal and conservative public figures of doing the bidding of the Muslim Brotherhood, and that mainstream Muslim-American organizations are actually fronts for foreign jihadi networks.

The theory of 'civilizational' or 'stealth jihad' argues "that Muslims are going to immigrate to the United States and gain access to various levers of control, especially in Washington," said Nathan Lean, author of The Islamophobia Industry: How the Right Manufactures Fear of Muslims. "And will ultimately engage in the grand destruction of the United States and the usurpation of the Constitution."

This theory flies in the face of the research: the conservative Manhattan Institute has documented that Muslim immigrants in America are extremely well-assimilated.

But Gaffney and others within the Islamophobia network, as critics have dubbed it, have seen their influence grow, and penetrate into the mainstream. While he was still running for president, Ben Carson repeated a central claim of the network, that Islam encourages believers to lie in order to achieve their goals.

The network is well-funded: according to the Center for American Progress, it draws upon tens of millions of dollars in foundation funds. The Council on American-Islamic Relations (a frequent target of anti-Muslim activists) has estimated that between 2008 and 2013, private donations brought the figure closer to $200 million.

Sometimes the attention has backfired. Last year, after the deadly Charlie Hebdo attacks in Paris, Steven Emerson of the Investigative Project on Terrorism went on Fox News and claimed that Europe was filled with 'no-go zones,' each of which function as a "country within a country" and is allegedly off-limits to the police. The claim was made repeatedly on the network before an international backlash prompted Fox News to issue a retraction and apologize.

"American Islam bashers talk about the alleged no-go zones in Europe as 'our future,'" said Mark Potok, a Senior Fellow at the Southern Poverty Law Center and a leading expert on extremism and hate. "And, in fact, a number of them have even made the claim that there are no-go zones in Dearborn, Michigan, which is, of course, the largest concentration of Muslims in the United States."

Even though it's "utterly false,"  Potok said, the rhetoric comes against a backdrop of actual terrorism, in Brussels, San Bernardino, Istanbul and other cities. He worries about the effect Cruz and Trump are having at a precarious moment.

"The fact is, there are millions and millions of people who are frightened. And this kind of talk frightens them more. And frightened people often do very dangerous, and bad things."