Kamala Harris is in a bind. Despite rallying Democrats behind her, she’s still being heckled by protesters who want to end U.S. support for Israel’s war in Gaza. Many of those activists want her to endorse an arms embargo against the Jewish state. Her chief foreign policy adviser, Phil Gordon, has ruled that out. But a flat refusal risks alienating progressives in key states like Michigan and sparking an ugly confrontation at this week’s Democratic convention.

There’s a solution that allows Ms. Harris to go beyond merely calling for a cease-fire and saying that “far too many” civilians in Gaza have died. Without supporting an arms embargo, she can still signal a clear break with Joe Biden’s near-unconditional support for an Israeli war effort that many legal scholars believe has led to genocide. And she can do so in a way befitting a former prosecutor: When it comes to Israel, Ms. Harris should simply say that she’ll enforce the law.

The law in question has been on the books for more than a decade. It prohibits the United States from assisting any unit of a foreign security force that commits “gross violations” of human rights. Aid can be reinstated if the foreign country adequately punishes the perpetrators. Passed by Congress in 1997, it bears the name of former Senator Patrick Leahy — and it has been applied hundreds of times — including reportedly against U.S. allies like Colombia and Mexico.

But it has never been applied to Israel, the country that over the past eight decades has received more U.S. aid, by far, than any other. That’s not because the Israel Defense Forces don’t commit serious abuses. “There are literally dozens of Israeli security force units that have committed gross violations of human rights” and should thus be ineligible for U.S. aid, a former State Department official, Charles Blaha, told ProPublica in May.

Mr. Blaha’s should know. From 2016 to 2023, he oversaw the office charged with enforcing the Leahy law. While a U.S. State Department spokesman in April claimed that Israel receives “no special treatment” under the Leahy law, Mr. Blaha says his own experience proved otherwise. When it came to every country except Israel, he has explained, career officials generally had the last word. In the case of Israel alone, he says, the decision rested with the State Department’s top political appointees.

Those appointees are failing to enforce U.S. law. This spring, ProPublica reported that an expert State Department panel had recommended that Secretary of State Antony Blinken cut off assistance to several units of the Israeli military and police after reviewing allegations that they had committed human rights abuses preceding the current Gaza war. In May, Mr. Blinken told Congress that Israel had adequately punished the members of units accused of serious abuses, and U.S. aid would thus keep flowing. (Because the department’s vetting process is not public, it’s unclear whether any of the units Mr. Blinken cleared were among those the panel flagged.)