The president continued his attacks on Monday, attempting to disparage House Speaker Nancy Pelosi in the process. “Before Nancy, who has lost all control of Congress and is getting nothing done, decides to defend her leader, Rep. Omar, she should look at the anti-Semitic, anti-Israel and ungrateful U.S. HATE statements Omar has made,” he said on Twitter. “She is out of control, except for her control of Nancy!”

Crenshaw may have sparked these attacks, but the president’s intervention has escalated the situation to something potentially dangerous, which is why — after a slow start — Democrats have begun to condemn the president’s actions, defending Omar by name.

“Ilhan Omar is a leader with strength and courage. She won’t back down to Trump’s racism and hate, and neither will we,” said Senator Bernie Sanders of Vermont. “The disgusting and dangerous attacks against her must end.”

Likewise, Senator Elizabeth Warren of Massachusetts accused Trump of “inciting violence against a sitting Congresswoman — and an entire group of Americans based on their religion.” That, of course, is the idea. Trump believes he benefits from the passions and anger stirred by racist demagoguery.

It is easy to tie these attacks to Trump’s history of anti-Muslim rhetoric. But anti-Muslim prejudice was common in Republican politics before he stepped on the political stage with his “birther” charges against President Barack Obama.

It was an important force among Republican voters — in one 2004 poll, for example, about 40 percent of self-identified Republicans said that Muslim Americans should be required to register with the government and 41 percent said that Muslim-American civic groups should be infiltrated by the government. Well before Obama was a household name and Trump a political figure, a 2006 Gallup poll found wide anti-Muslim prejudice “with Republicans ascribing more negative political and religious qualities to Muslims, and being more opposed to having Muslims as neighbors than are Democrats and independents.”

It was an important force in conservative media. Conservative radio and television hosts frequently conflated all Muslims with the actions of extremists. In one 2006 segment on his radio show, Glenn Beck warned that if “good Muslims” aren’t “the first ones in the recruitment office lining up to shoot the bad Muslims in the head,” then “human beings” might be forced into “putting up razor wire and putting you on one side of it.”