PRESIDENT TRUMP has trampled on innumerable American democratic norms and traditions, but few instances have been as egregious as his public intervention Thursday urging Israel to deny entry to a pair of U.S. members of Congress who planned to visit the country this weekend. Hours later, Israeli Prime Minister Benjamin Netanyahu, whose ambassador in Washington said last month that the Jewish state would never bar a member of Congress “out of respect” for the institution and the Israeli-American alliance, did just that.

The episode is a disgrace to both countries and a demonstration that craven domestic political self-interest drives each leader far more than principle or the underlying importance of what until now has been an enduring alliance. By teaming up with Mr. Netanyahu to block the visit by Reps. Ilhan Omar (D-Minn.) and Rashida Tlaib (D-Mich.), Mr. Trump demonstrated contempt for the congresswomen and their constituents. That’s old news for a president who said the two legislators and two other minority Democratic congresswomen, who are all Americans, should “go back” to their countries. What’s added in this case is the odious assertion that their freedom to travel should be curtailed because of their political beliefs.

Mr. Trump delights in fanning the flames of racial and ethnic division, and in Ms. Omar and Ms. Tlaib he has found foils that suit his purpose. Both women favor the BDS movement, which advocates boycotts, divestments and sanctions directed at Israel for its occupation of the West Bank and violations of Palestinian human rights. Both have made statements that play on anti-Semitic tropes. Both have broken with the Democratic Party’s broad tradition of support for Israel — which Mr. Trump hopes gives him an opening to drive a wedge between the party and its base of Jewish voters.

As for the principle that elected representatives of the American people should be able to travel where they like — the principle enunciated a few weeks ago by Ron Dermer, Israel’s ambassador in Washington — Mr. Trump has made his rejoinder clear: Principles are for suckers. How many dictators are rejoicing today that a U.S. president has given them full permission to bar members of Congress who in the future might want to visit their countries to monitor elections or speak up for human rights?

Then there’s Mr. Netanyahu, who has dealt a blow to the U.S.-Israeli alliance in service to his prospects of winning a fifth term in next month’s Israeli election. His decision might appeal to his hard-line supporters at home. But as a former diplomat who worked extensively in the United States, he knows that Israel’s interests depend on cultivating strong ties with Republicans and Democrats alike. That stance was already strained by his hostile relationship with President Barack Obama. Now, by barring the two Democratic representatives, he has antagonized a broad swath of American Democrats who might not embrace their view of Israel but will be justly furious at the insult meted out, gratuitously, by an Israeli premier.

Israel enacted a law two years ago authorizing officials to deny entry to supporters of the BDS movement. That was a sign of weakness: How could a country with a robust pluralistic democracy bar nonviolent visitors based on their political beliefs? That a U.S. president would lend support and credence to such a policy — at the expense of democratically elected members of Congress — is fundamentally un-American.

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