President Trump turned professional sports into a political battleground Friday night into Saturday, directing full-throated ire toward African American athletes who have spoken out against him and prompting a sharp rebuttal from the National Football League and several prominent sports figures, including the first Major League Baseball player to kneel during the national anthem .

In a span of roughly 12 hours, as the sports world would typically be gearing up for college football and baseball’s pennant races, Trump ensnared and agitated the most powerful sports league in North America and angered NBA superstars Stephen Curry and LeBron James. His comments set the stage for potential mass protest Sunday along NFL sidelines.

At a political rally Friday in Huntsville, Ala., Trump called on NFL owners to release players who demonstrated during the national anthem in the manner of former San Francisco 49ers quarterback Colin Kaepernick, who knelt last season to draw attention to police violence against African Americans. Saturday morning on Twitter, Trump rescinded a White House visit invitation to Curry of the NBA champion Golden State Warriors, although it is unclear whether the Warriors had been invited in the first place.

“Going to the White House is considered a great honor for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!” Trump posted at 7:45 a.m. Saturday.

The tweet came on the heels of comments he made Friday night at a rally for Republican Sen. Luther Strange, who is running in a special GOP primary election to remain in the seat vacated by Attorney General Jeff Sessions.

“Wouldn’t you love to see one of these NFL owners, when somebody disrespects our flag, to say, ‘Get that son of a bitch off the field right now. Out! He’s fired. He’s fired!’ ” Trump said. “You know, some owner is going to do that. He’s going to say, ‘That guy that disrespects our flag, he’s fired.’ And that owner, they don’t know it [but] they’ll be the most popular person in this country.”

Saturday afternoon, Trump doubled down on Twitter.

“If a player wants the privilege of making millions of dollars in the NFL, or other leagues, he or she should not be allowed to disrespect our Great American Flag (or Country) and should stand for the National Anthem,” Trump wrote. “If not, YOU’RE FIRED. Find something else to do!”

NFL Commissioner Roger Goodell, who has waffled in either supporting or decrying Kaepernick, responded to Trump in a statement released Saturday morning.

“Divisive comments like these demonstrate an unfortunate lack of respect for the NFL, our great game and all of our players, and a failure to understand the overwhelming force for good our clubs and players represent in our communities,” Goodell said.

Saturday evening, Trump responded once again on Twitter.

“Roger Goodell of NFL just put out a statement trying to justify the total disrespect certain players show to our country,” Trump said. “Tell them to stand!”

As of late Saturday afternoon, NFL players were still deciding if and how they would respond Sunday. Many players felt it would be difficult to organize a leaguewide protest on short notice, especially because Trump’s volatility made the situation tenuous.

“Guys got hit in the face [Friday] night,” said one person familiar with the NFL Players Association’s thinking, who requested anonymity to discuss a sensitive issue.

Many players called the union to ask what kinds of protests are permissible under league rules, according to the person. Some, for example, wanted to know if they could wear a T-shirt over the shoulder pads during warmups, but that would be a violation of NFL equipment rules.

Some players also worried Trump had put them in a bad position: If they protested, Trump could reassert talking points to his base. If they observed the anthem, Trump could claim responsibility.

“Comments like we heard from the president are inappropriate, offensive and divisive,” two of the NFL’s most prominent owners, John Mara and Steve Tisch of the New York Giants, said in a statement. “We are proud of our players, the vast majority of whom use their NFL platform to make a positive difference in our society.”

San Francisco 49ers owner Jed York, the man who last employed Kaepernick, called Trump’s comments “callous and offensive” and “contradictory to what this country stands for.”

Various NFL owners collectively donated more than $7 million to Trump’s presidential campaign, and New England Patriots owner Robert Kraft gave Trump a Super Bowl ring this summer. Saturday, a Patriots spokesman did not respond to a request for comment from Kraft.

“In calling upon his NFL ownership cronies to ‘fire the SOBs,’ he has effectively thrown these owners under the bus in exchange for a moment of applause in Alabama,” said Harry Edwards, a sociology professor at the University of California Berkeley. “Every owner, and especially the seven who supported him with both money and public association, are going to have to answer the questions: ‘What side of history are you on? Do you agree with Trump?’ If they agree or have no comment, they will be aligned against both the NFL commissioner and league office and the NFLPA. If they do not agree with his Alabama statements, they will in effect have separated themselves from both Trump and his alt-right constituency.”

Once again, Trump has placed himself squarely at the center of a wrenching national debate over race. But unlike past presidents who have given at least some voice to a desire to bridge the historic divides in American life, Trump seems eager to lean into those disputes.

For years as a private citizen, Trump was the most vocal proponent of the falsehood that President Barack Obama was not born in the United States, a racially tinged conspiracy theory that thrived in fringe corners of the right. And during the presidential campaign, he was accused by his opponents of using his social media account to amplify anti-Semitic and racist voices.

As president, facing his first major test on issues of race, Trump fumbled. After a group of white supremacists and neo-Nazis descended upon Charlottesville to rally in favor of preserving confederate monuments, Trump was widely condemned for arguing that the violence that ensued was the result of “both sides,” including counterprotesters who had taken to the streets to oppose the racist marchers.

“Trump has no interest in trying to bridge racial tensions,” said Tim Miller, a Republican operative. “He wants to play into white grievance policies. He wants to continue to divide the country further and to expand the wedge.”

Miller added: “It’s particularly the case on racial issues; Donald Trump is obviously more animated by perceived reverse racism and white grievance than he is by other racial justice issue in this country.”

For many African Americans, the trend has left no doubt about Trump’s true feelings on matters of race.

“This is the biggest white guy in the world trying to take on black America in total,” said Michael Eric Dyson, a professor of sociology at Georgetown University.

At the Huntsville rally, Trump called for fans to boycott the NFL in the face of player anthem protests after bemoaning rules penalizing dangerous hits.

“But do you know what’s hurting the game more than that?” Trump said. “When people like yourselves turn on television and you see those people taking the knee when they’re playing our great national anthem. The only thing you could do better is if you see it, even if it’s one player, leave the stadium. I guarantee things will stop. Things will stop. Just pick up and leave. Pick up and leave. Not the same game anymore, anyway.”

Despite a clear case that his performance and ability warranted at least a chance at a roster spot, Kaepernick remains unsigned.

At the rally, Trump went on to criticize the NFL for rules meant to improve player safety.

“The NFL ratings are down massively,” he said. “Now, the No. 1 reason happens to be they like watching what’s happening with yours truly. They like what’s happening. Because you know today if you hit too hard — [game officials penalize the offending team] 15 yards! Throw him out of the game! . . . They’re ruining the game! That’s what they want to do. They want to hit. They want to hit! It is hurting the game.”

DeMaurice Smith, the executive director of the NFL Players Association, said in a tweet early Saturday morning, “The union . . . will never back down when it comes to protecting the constitutional rights of our players as citizens as well as their safety as men who compete in a game that exposes them to great risks.”

Trump’s veering into sports Friday night comes after months of simmering tension between high-profile sports figures and the president. Warriors Coach Steve Kerr and San Antonio Spurs Coach Gregg Popovich both offered stinging critiques of Trump after his election. Many Patriots skipped the White House visit after winning February’s Super Bowl. A raft of athletes, University of Virginia alums Washington Nationals reliever Sean Doolittle and Philadelphia Eagles defensive end Chris Long among them, blasted Trump’s response to the violence in Charlottesville.

Saturday night, Oakland Athletics catcher Bruce Maxwell, a 26-year-old rookie and the son of an Army veteran, became the first Major League Baseball player to kneel during the national anthem. Teammate Mitch Canha stood next to him with his hand on Maxwell’s shoulder and hugged Maxwell afterward.

Curry, the star of the NBA champion Warriors, attracted the president’s attention Friday afternoon. The Warriors announced they would vote as a team whether to attend the White House, as is tradition for champions from all sports. Curry said he would vote against going.

“We don’t stand for basically what our president . . . the things that he said and the things that he hasn’t said in the right terms that we won’t stand for it,” Curry said. “And by acting and not going, hopefully that will inspire some change when it comes to what we tolerate in this country and what is accepted and what we turn a blind eye to.”

Curry said Saturday that he woke up to 20 text messages, then discovered why the messages had come when he checked Twitter.

“It’s surreal, to be honest,” Curry said. “I don’t know why he feels the need to target certain individuals rather than others. I have an idea of why, but it’s kind of beneath a leader of a country to go that route. That’s not what leaders do.”

Presidential White House invitations to championship teams are typically straightforward, celebratory affairs. In 2016, President Obama called Coach Tyronn Lue four days after the Cleveland Cavaliers won the title to offer congratulations and invite them to the White House. Administration officials and team executives then worked out scheduling details.

After Trump said he rescinded Curry's invitation, LeBron James slammed Trump on Twitter.

“U bum @StephenCurry30 already said he ain’t going!” James said. “So therefore ain’t no invite. Going to White House was a great honor until you showed up!”

In a video uploaded later Saturday to the social media product Uninterrupted, James further criticized the president.

“I think it’s basically at a point where I’m just a little frustrated,” James said. “Because this guy that we’ve put in charge has tried to divide us once again, and obviously we all know what happened with Charlottesville and the divide that caused.”

James’s statements illustrated a new paradigm in athlete activism. In the past, athletes have commented on the actions of politicians. This weekend, they are acting on the comments of a politician.

“The players are asking the right question,” University of Richmond Law School Chair Carl Tobias said. “When we speak out about political issues, why are we told to stick to sports?”

Tim Bontemps contributed from Oakland, Calif.