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.”
For candidates competing in a wide open presidential primary, the question of how — and when — to engage with Mr. Trump’s political broadsides has become a dividing line among Democrats and party activists.
Just as some advocacy groups are looking to make certain policies litmus tests for a candidate’s progressive credentials, such as “Medicare for all” and the Green New Deal, others are monitoring how quickly and forcefully each responds to Mr. Trump’s most inflammatory moments — whether he’s using images from the nation’s darkest days to attack Ms. Omar or suggesting that authorities should release detained migrants in Democratic cities, as he also did in recent days.
Mr. Trump’s actions do not divide Democrats on the merits, as nothing unites all corners of the party more than the desire to oust him from the White House. His effect on the primary is more political; for voters and activists outraged by Mr. Trump’s actions and rhetoric, only a quick and unequivocal response is satisfactory.
Mr. Trump set off the controversy when he targeted Ms. Omar on Friday with a video of the World Trade Center burning and other images from the attacks, playing to suggestions from conservatives that she had minimized the horrors of that day in a speech she gave last month. Ms. Omar’s spokesman said the right was mischaracterizing her remarks.
In supporting Ms. Omar over the weekend, the differences in the statements from Democrats were slight, but striking. Some candidates, such as Ms. Warren and Ms. Harris, overtly tied Mr. Trump’s actions to xenophobia and an anti-Muslim sentiment. “I’ll be blunt — we must defeat him,” Ms. Harris said.
“I am grateful for @IlhanMN courage and leadership and I stand with her — and with others targeted by the President’s anti-Muslim rhetoric,” tweeted another candidate, former Housing Secretary Julián Castro.
Candidates who have tried to position themselves as pragmatic options to a party lurching to the left, including Mr. O’Rourke, Senator Amy Klobuchar of Minnesota and Representatives Tim Ryan of Ohio and Eric Swalwell of California, all offered varying measures of support, but also tried to refocus the incident to highlight the need to unify the country.
Mr. O’Rourke was criticized for an initial statement that did not mention Ms. Omar by name, but rather said: “We are stronger than this president’s hatred and Islamophobia. Do not let him drive us apart or make us afraid.”
Later, on the campaign trail, Mr. O’Rourke updated his rhetoric to match the calls from Democratic activists, invoking Ms. Omar specifically and connecting Mr. Trump’s tweets to controversial policies such as family separation at the border and the administration’s travel ban for predominantly Muslim countries.
“This is an incitement to violence against Congresswoman Omar and our fellow Americans who happen to be Muslim,” Mr. O’Rourke said later.
“This hatred and division and paranoia and anxiety sown by this president are dangerous, and the consequences frankly are mortal for our fellow Americans,” he added. “He is using the oldest play in the book: fear and division.”
Zahra Billoo, the executive director of the Council on American-Islamic Relations chapter in the San Francisco Bay Area, said that, “as a Muslim and a woman of color and a daughter of immigrants,” these moments help give her clarity on who is willing to stand up for her community.
While acknowledging that most voters won’t be aware of the fine gradations between the various statements, or the times they were released, Ms. Billoo said the moments in aggregate can help create an impression about which candidates can be trusted by historically marginalized communities.
“We are seeing a shift in the Democratic Party where there’s a fracture,” Ms. Billoo said in a phone interview. “One side being moderate, even timid. And the emerging side of the party is stepping up to the plate, and recognizing that we’re in an unprecedented moment in time.”
“I’m more invested in the candidates who are willing to speak up and call it what it is,” she said.
The statement from Ms. Gillibrand, in particular, raised eyebrows in politically active circles. Though she has sought to cast herself as full-throated progressive, she qualified her support for Ms. Omar by saying, “As a Senator who represents 9/11 victims, I can’t accept any minimizing of that pain.” Critics lashed back, saying her comments only gave more credence to the slew of misinformation surrounding Ms. Omar.
Ms. Epps-Addison called it shameful, as did Ms. Billoo.
Diane Alston, a 24-year-old from Houston who had recently been quoted in an article about the New York senator’s most vocal superfans, nicknamed “Gillistans,” said the statement was enough to force her to rethink whom she planned to support.
“Ilhan didn’t minimize the pain of 9/11 victims,” Ms. Alston said.
“For Gillibrand to state that she did — when her words were blatantly taken out of context — is shameful and legitimizes a smear against a black Muslim woman already facing death threats,” she said.
Tyler Law, a party strategist and former spokesman for the Democratic Congressional Campaign Committee, said the weekend should be instructive for all candidates: In an election against Mr. Trump, they will have to be prepared to deal with his shameless politicking, in which he will use his large platform to amplify attacks through the media.
“The way this clip was laundered through the press is almost a perfect master class,” Mr. Law said. “The entire media ecosystem our outrage thrives in is so clearly not built for the new perspectives and nuances of the most diverse Democratic caucus in history.”
“The 2018 midterms demonstrated clearly that while voters are not eager to support someone who will evoke Trump’s name at every turn,” Mr. Law said, “they sure as hell want someone with the backbone to stand up to this bully.”
Speaker Nancy Pelosi and Tom Perez, chairman of the Democratic National Committee, have both been criticized for not standing with Ms. Omar more unequivocally.
On Sunday afternoon, Ms. Pelosi announced that she had instructed congressional officials to do a “security assessment” to ensure that Ms. Omar was safe following Mr. Trump’s Twitter post.
“The president’s words weigh a ton, and his hateful and inflammatory rhetoric creates real danger,” she said. “President Trump must take down his disrespectful and dangerous video.”
On Sunday afternoon, it remained on the president’s page.