The first reaction came Friday evening, from Senator Bernie Sanders of Vermont.

President Trump had escalated his political attacks on Ilhan Omar, the Minnesota Democrat who is also one of the first Muslim women to serve in Congress, tweeting an inflammatory video that implied she trivialized the horror of the Sept. 11 terror attacks. Ninety minutes later, Mr. Sanders called the video “disgusting and dangerous,” and an example of “Trump’s racism and hate.”

It kicked off a cascade of analogous statements from other Democrats who are running for president, including Senator Elizabeth Warren, Pete Buttigieg and former Representative Beto O’Rourke, all of whom weighed in shortly afterward.

Not on that list: Senators Kamala Harris, Kirsten Gillibrand and Cory Booker, who waited until later in the weekend to offer statements of support.

It did not go unnoticed.

“Black folks are watching. Muslim folks are watching. Brown folks are watching,’’ said Jennifer Epps-Addison, the co-executive director of the Center for Popular Democracy, a liberal organizing group focusing on minority communities. “And we’re making our decisions about who to support in real time. When your sister is being attacked, you can’t wait to get the politics right.”