Hear Why Laura Ingraham Foresees a GOP Convention 'Crisis'

WATCH: Trump Tells Greta Why He's Proposing Total Ban on Muslims Entering U.S.

Donald Trump is at the top of the news cycle - and generating a huge backlash - once again after he called for a ban on Muslim immigration to the U.S.

Trump said on On the Record that the move is necessary after the San Bernardino terror attack due to the "tremendous animosity" by some Muslims towards Americans.

But, is that even legal under the Constitution?

Judge Andrew Napolitano said on Fox Business Network that it would not be allowed under the Constitution. 

Napolitano said if Congress were to pass a law outlining a "religious test" for those seeking to come to the U.S., "the courts would invalidate it before any meaningful enforcement took place." 

He said the basic freedom of religion is enshrined in the First Amendment. 

Napolitano said Congress could, however, pass a law banning immigration from a specific country. 

Watch more from Judge Napolitano above.

Judge Nap: Family of Calif. Attackers Could Be Charged for Failing to Report Treason

Air Force Football Players' On-Field Prayers Called 'Putrid,' 'Disgrace'

Baby Survives After Being Buried Alive Along Bike Path