Mr. Trump’s aim of stoking an endless culture war puts his political critics in a bind. They can take his bait and fight back, participating in the divisive distraction he’s designed to energize his supporters, or they can ignore his outbursts and risk normalizing his terrible behavior.

Mr. Trump, who has never shown any interest in winning over most Americans, does everything he can to harden divisions by enraging his opponents. In another Monday tweet, he dared Democrats “to unite around the foul language & racist hatred spewed from the mouths and actions of these very unpopular & unrepresentative Congresswomen.” A few hours later, the House speaker, Nancy Pelosi, announced that the chamber would introduce a resolution condemning Mr. Trump’s latest affront. In a letter to Democratic members, she asserted: “The House cannot allow the president’s characterization of immigrants to our country to stand. Our Republican colleagues must join us in condemning the president’s xenophobic tweets.”

If only. While Democratic criticism can wind up serving Mr. Trump’s narrow political purpose, rebukes from Republicans would undercut it and, possibly, chasten him. But most Republican leaders appear to be either too delighted by his conservative judicial picks or his deregulatory agenda or too afraid of his impassioned following to speak up. They may well recognize and reject the corrosiveness of his ethnonationalism, they may fear its impact on the country and their party, but most have shown over and over that they won’t do a thing about it.

This time, a few Republicans have summoned the nerve to express their disapproval. A representative from Texas, Will Hurd, the only Republican representing a border district, called Mr. Trump’s tweets “racist and xenophobic” and “unbecoming of the leader of the free world.” Other early objectors included Representative Pete Olson , also of Texas, who urged the president to “immediately disavow his comments,” and Representative Susan Brooks of Indiana, who is not seeking re-election and called the remarks “inappropriate.” Among the harshest rebukes came from Representative Mike Turner of Ohio, who deemed the president’s remarks “racist” and called for him to apologize.

Less courageous was Senator Susan Collins of Maine, who issued a statement that led with strong criticism of the Democratic congresswomen attacked by Mr. Trump and closed with a weak plea for the president to delete his “way over the line” tweets.