Responding to criticism of his latest remarks, he said Wednesday that his talk “is only anti-Semitic in your head.” But that’s precisely where anti-Semitism resides: in the minds of people who see Jews as somehow alien.

Mr. Trump, who has an undoubted eye for the weaknesses of his political opponents, is seeking to exacerbate emerging divides in the Democratic Party over America’s alliance with Israel. The dangers extend well beyond his recourse to anti-Semitic statements.

Even incremental movement toward polarizing Americans over Israel undermines the long-term security of the Jewish state, which has long rested on the firm foundation of bipartisan American support.

Mr. Netanyahu appears blind to this danger. He has chosen to reap the short-term benefits of Mr. Trump’s embrace, delighting in small victories like the decision to move the American Embassy to Jerusalem and, at Mr. Trump’s urging, preventing two Muslim congresswomen, Ilhan Omar of Minnesota and Rashida Tlaib of Michigan, from making an official visit to Israel.

Mr. Trump is also doing Israel no favors over the long haul by discarding long-running American efforts to address the hopes and grievances of Palestinians and Israelis through a negotiated deal.

For Democrats, the challenge is to resist the easy gratification of reflexively opposing what Mr. Trump supports — or supporting what he opposes.

Mr. Trump wants to keep Ms. Omar and Ms. Tlaib in the spotlight because they are outspoken critics of American support for Israel . Ms. Omar has invoked anti-Semitic language in support of her arguments. Both support the Boycott, Divestment and Sanctions movement, which is silent in its official documents about Israel’s right to exist, and which seeks to use economic pressure to extract concessions from Israel that the United States has long maintained should be negotiated as part of a peace agreement.