President Donald Trump can’t stop using — and abusing — his legal authority to block the entry of noncitizens into the country.

When he issued a travel ban on citizens of Muslim-majority countries early in his first term, he did so by invoking Section 212(f) of the Immigration and Nationality Act, which allows him to block any foreigner if he deems that their entry would be “detrimental to the interests of the United States.”

When he issued a proclamation turning away noncitizens who could not demonstrate the ability to pay for their health care costs, he cited Section 212(f).

When he halted most legal immigration at the beginning of the Covid-19 pandemic, first from China and then from other countries, there was Section 212(f) again.

Finally, last week, he announced that he would block foreign students from receiving student visas to attend Harvard and implement a travel ban on 12 countries, as well as restrictions on seven others. The travel ban took effect on Monday, just after midnight, and the legal framework for both orders was built on Section 212(f).

Some of Trump’s attempts to invoke Section 212(f) have been challenged in court. Judges struck down several versions of Trump’s........

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