NEWARK, N.J. — Republican presidential candidate Donald Trump has cited several statistics from a “highly respected” group to justify his proposal to temporarily bar Muslims from entering the country.

They include the assertion that 25 percent of American Muslims believe violence in the United States is justified as a part of a global jihad, and that 51 percent agree Muslims in America should have the choice of being governed by US or Islamic law.

But those figures come from a problematic June poll commissioned by a think tank that has long theorized that Muslims are trying to infiltrate the United States.

The Center for Security Policy was founded in 1988 by Frank Gaffney, who has warned that the Muslim Brotherhood — an Islamist political party now banned in its native Egypt — is infiltrating the federal government in an attempt to overthrow it and install Islamic law in the US.

Gaffney has called for formation of a new congressional committee charged with “examining and rooting out anti-American — and anti-constitutional — activities that constitute an even more insidious peril than those pursued by communist Fifth Columnists fifty years ago.”

His actions have drawn the scorn of his fellow conservatives, including the American Conservative Union, which barred Gaffney from speaking at its Conservative Political Action Conference after he accused two of its board members, including anti-tax activist Grover Norquist, of being compromised by the Muslim Brotherhood.

The center did not make Gaffney available for an interview. But spokesman Alex VanNess said in a statement that “the acceleration of terror attacks such as the recent attacks in Paris, Chattanooga and San Bernardino makes plain that it is both prudent and necessary to respond to the threat posed by jihadist terror in a way that both recognizes it and calls it what it is.”

The poll was conducted online by the conservative pollster The Polling Company, and it’s unclear how respondents were selected to participate. Many online surveys do not start with a random sample of the US population, and as such their results may not be representative of the population as a whole.

Some of the survey questions were also vaguely worded. For example, while many poll respondents said Muslims should have the option of having their own courts to apply Islamic law, it was unclear based on the question wording whether that would supplant or supplement US law, or under what circumstances Islamic law would apply.

Few poll respondents said Muslims should only be subject to Islamic law. Many respondents said they consider Islamic law, known as Shariah, to be mostly a personal moral code, with most saying it is compatible with the US Constitution.