In 1952, with the nation fearful of communist infiltration, Congress gave the president the authority under the Immigration and Nationality Act to take action:

“Whenever the president finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may … suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate,” the law says.

That power has been invoked dozens of times. But legal experts say those examples were more limited than what Trump has sought.

Citing a report that reviewed White House administrations going back to Reagan, Chuang noted in his ruling that no president has issued a ban on the entry “of all citizens from more than one country at the same time, much less six nations all at once.”

Chuang found that the travel ban likely violated another aspect of federal immigration law, barring discrimination on the basis of nationality in the issuance of immigrant visas. That law was passed in 1965 as part of an effort to end longstanding immigration quotas that had been criticized as racist.

Ultimately, the cases will come down to the ways in which that law and the Constitution constrain the president’s authority.

“That’s the tug of war that is going to play out and, I suspect, go before the Supreme Court,” said Ted Ruthizer, a former president of the American Immigration Lawyers Association. “I think it will be a very seminal decision as to what are the limitations on the executive’s powers.”

Thanawala reported from San Francisco. Associated Press writers Ben Nuckols in Greenbelt, Maryland; Jessica Gresko, Sarah Brumfield and Jill Colvin in Washington, D.C.; and Jennifer Sinco Kelleher in Honolulu contributed to this report.

This is breaking news. Please check back for updates.

— CNBC contributed to this report.

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