The Trump administration is asserting that it has broad power under a 1952 law to kick out foreign students who participated in pro-Palestinian protests. That statute says the secretary of state can deem noncitizens deportable for foreign policy reasons, and the secretary, Marco Rubio, made clear recently that he had already used it to cancel hundreds of student visas.

“It might be more than 300 at this point,” Mr. Rubio said last week. “We do it every day. Every time I find one of these lunatics, I take away their visa.”

But that expansive conception of power appears to conflict with a key limit Congress added nearly four decades after the law passed. Lawmakers explained that the modification, which is recorded elsewhere in federal statute books, means the law may be used “only in unusual circumstances” and “sparingly” if the problem stems from foreigners’ exercise of free speech.

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