Republican nominee Donald Trump's proposal to ban Muslims from entering the United States met with strong opposition from Democrats, American Muslims, and now immigration experts have piled on.

"You are never going to be able to block the lawful migration of people based on their religious views," John Sandweg, former acting director of U.S. Immigration and Customs Enforcement, told NBC News.

According to Sandweg, instituting a ban on Muslims is "impossible."

"The volume is probably too significant" for a ban, says Dan Crocetti, creator of the Fraud Detection and National Security Directorate of the U.S. Citizenship and Immigration Service.

"I don't think it's doable," said Michael Cutler, a former Immigration and Naturalization Service special agent with 30 years experience. NBC cited federal estimates that indicated 189 million people entered the U.S. legally last year alone.

Problems implementing Trump's proposal stem first from how Muslims would be screened. Only a few nations, including Jordan, Turkey and Pakistan, keep records on their citizens' religion, while seven countries with the largest Muslim populations in the world don't keep track.

Trump, in an MSNBC interview, has suggested that customs agents ask, "Are you a Muslim?" and turn away those who say yes.

"The difficulty is if people lie," said Doris Meissner, a former INS commissioner and a senior fellow at the Migration Policy Institute. "I don't know how you would check against that — it is impractical."

Despite its challenges, a majority of Americans support a ban on Muslims traveling to the U.S., according to polling data from The Washington Post.

The expense of a ban, which Trump says would save billions, would cost anywhere from $40 billion to $60 billion or more, just to cover screening tests for Muslim immigrants.

Marketplace estimates that the U.S. would lose $18 billion annually from Muslim tourism, and $5 billion from Muslim students who would have studied in America.

"The first and foremost cost is it's going to flush the Bill of Rights on day 1 of any potential Trump presidency," Corey Saylor, director of the Council on American Islamic Relations' office that monitors Islamophobia, told Marketplace.

"We will fall right off the stage as the leader of the free world."