By Sunday, the sports-watching world was confronted with a display of politics in the athletic arena with no recent precedent in the United States — from images of two football squads declining to take the field in Nashville; to a New York Giants star, Odell Beckham Jr., celebrating a touchdown by raising his fist to the air; to a procession of football executives, including some personal friends of Mr. Trump, expressing unease with his remarks.
The Rev. Jesse L. Jackson said Mr. Trump had effectively challenged athletes of all races to rise against him, by using language Mr. Jackson described as displaying a “slave-master-servant mentality.” Mr. Jackson said it was incumbent on athletes, irrespective of their race, to show Mr. Trump they could not be belittled or stripped of their right to free speech.
“They should all kneel, not against the flag, but against the interference by Mr. Trump with their First Amendment rights,” Mr. Jackson said, urging players not to underestimate their power: “If the cotton pickers don’t pick cotton, the industry doesn’t move; the N.F.L. and N.B.A. players don’t play the game, it doesn’t move.”
Mayor Bill Peduto of Pittsburgh, whose city saw its beloved Steelers football team stay in the locker room during the national anthem at a game in Chicago on Sunday, described Mr. Trump’s remarks as alarming. Mr. Peduto, who said he supported the Steelers’ protest, said he planned to hold discussions on the local level with residents who feel personally wounded by the controversy.
“If you’re a gold star mom, the idea of kneeling through the national anthem is beyond disgraceful and is a cause of emotional harm,” Mr. Peduto said. “But if you’re a mom who lost their child in the streets of America, the idea of kneeling is saying to you that your voice is being heard.”
In his city, Mr. Peduto said, “there are two distinct sides that need to have a conversation, not a president who chooses sides.”
Mr. Trump is unlikely to back down in the face of criticism. He has intensified his statements repeatedly since declaring Friday in Huntsville, Ala. that N.F.L. owners should fire any “son of a bitch” who fails to join displays of patriotism, a clear allusion to Mr. Kaepernick’s practice of kneeling during the national anthem to protest police brutality.
The president followed up on Saturday by disinviting Mr. Curry’s entire team to the White House, after Mr. Curry said he would not visit the presidential residence to celebrate the Warriors’ basketball championship. And Mr. Trump suggested on Twitter on Sunday that football fans might “refuse to go to games until players stop disrespecting our Flag & Country.”
Speaking to reporters on Sunday, Mr. Trump said his criticism had “nothing to do with race.”
Mr. Trump has a long record of wielding racial and cultural divisions to his political advantage, as well as making inflammatory comments that distract from his policy agenda or from problems he’d prefer the news media not cover. His statements this weekend, for instance, drew attention away from a flagging effort by Senate Republicans to repeal the Affordable Care Act.
He has been applauded by some elected Republicans, enthusiastically so by many in the conservative news media. Laura Ingraham, a radio and television host who is closely aligned with Mr. Trump, cheered his rebuke of Mr. Curry on Twitter: “Pro-athletes who can’t set aside politics” to visit the White House, Ms. Ingraham said, should “be treated like the spoiled children they are.”
But Mr. Trump may also have risked a more complicated backlash than he is accustomed to. The episode follows close on the heels of Mr. Trump’s comments in August equating anti-racism protesters with neo-Nazis, a moment that threatened to destabilize his presidency. And professional athletes are not a traditional punching bag for Republicans, like Hollywood actors or the news media.
With the advent of Twitter and other social media, admired athletes like Mr. Curry and LeBron James — who chastised Mr. Trump on Saturday — have vast personal followings with whom they can communicate directly, bypassing owners and the news media, unlike sports heroes of the past.
Former Senator Bill Bradley, a Democrat and retired New York Knicks star, urged athletes to do just that on Sunday, calling in a statement for “everyone who has ever been a fan or a player, as a kid or as an adult” to reject Mr. Trump’s comments.
Perhaps the most significant warning sign for Mr. Trump was the sometimes-strained criticism from football team owners, a largely white and Republican-leaning group that includes several personal associates of the president. Mr. Trump collected support during the 2016 campaign from an array of sports executives and athletes, despite the racially divisive nature of his campaign.
But by Sunday, several owners who have been prolific donors to Mr. Trump and the Republican Party had spoken out against him in cautious language, including Robert C. McNair, owner of the Houston Texans, and Robert K. Kraft of the New England Patriots, a close friend of Mr. Trump.
“The comments made by the President were divisive and counterproductive to what our country needs right now,” Mr. McNair said in a statement.
Ari Fleischer, a former White House press secretary who advises athletes and sports teams on public relations, said Mr. Trump’s acidic language and calls for retribution may have undermined public sympathy for his most basic demand — that athletes stand for the national anthem.
Public opinion polls, Mr. Fleischer noted, have found Americans generally unfriendly to the idea of protesting during the anthem.
“He has created more kneelers today, which I regret,” Mr. Fleischer said of the president. “Trump went too far, but the overwhelming majority of Americans do not want to see football players disrespect the national anthem.”
Still, for all the immediate blowback against Mr. Trump, it remains to be seen whether the tumultuous weekend might herald a more lasting cultural shift in sports toward more overt acknowledgment of racial and political issues. Most athletes and team owners have typically avoided explicit political involvement, fearing that they might alienate fans who hold a range of views.
The sudden rallying around Mr. Kaepernick, several activists noted, came well after the quarterback came under ferocious criticism last year for his form of protest. Mr. Kaepernick is currently unsigned and appears not to have been hired by any team due to the controversy surrounding him.
Invoking Baltimore’s football team, Benjamin T. Jealous, a former president of the N.A.A.C.P., who is running for governor of Maryland as a Democrat, said, “I’d be very proud if the Ravens chose to give Kaepernick another shot.” He said Mr. Trump’s attacks had forced a more forthright reckoning with the rights of athletes to speak their minds.
“He ultimately challenged each of us to stand up and be very clear about what our flag stands for and what we believe,” Mr. Jealous said.