Some members of the San Francisco 49ers kneel during the national anthem before the first half of their Oct. 1 game against the Arizona Cardinals. (Rick Scuteri/AP)

The controversy over football players kneeling in protest during the national anthem could have simply remained a labor dispute within the NFL. But then President Trump tweeted that tax breaks should be revoked for a league that disrespects “our Anthem, Flag and Country.”

Those words threatening government action to financially penalize the league injected a new dimension into a roiling debate over race, police brutality and free speech that has gripped America’s most successful sports business for more than a year.

The National Football League is expected to decide this week whether to force players to stand for the national anthem. Legal experts say that NFL players, as employees of private teams, do not have First Amendment protections against the league and would not ordinarily be able to challenge that decision on free-speech grounds.

But Trump’s intervention — through last week’s tweet and its implied threat that the government would change tax laws to hurt the NFL — could provide the players with a stronger legal basis for a free-speech challenge against the United States, some legal scholars said. Left unchecked, Trump as president could financially compel any number of companies and private entities to do exactly what he wants, they said.

“The biggest wild card of all here is the president’s tweets,” said Marc Edelman, who teaches sports law at Baruch College in New York. “The NFL didn’t publicly voice ­opposition until baited into doing so and being threatened with financial sanctions by the president of the United States.”

On Sunday, seven San Francisco 49ers players knelt during the anthem before the game against the Washington Redskins at FedEx Field in Landover, Md. — a small fraction of the number who protested during last week’s 49ers road game against the Indianapolis Colts, a demonstration that prompted a walkout from Vice President Pence. There were scattered protests across the league Sunday.

The players' union has not decided whether it will challenge the NFL if the league decides at a meeting of team owners, set to begin Tuesday, to require all players to stand for the anthem.

The American Civil Liberties Union told The Washington Post that it stands ready to defend, in principle, the rights of players to express themselves and to challenge Trump's "unconstitutional efforts to bully the NFL into complying with his view of what is politically correct."

If the NFL acts because of Trump’s threat to punish the league, players could legitimately claim that their First Amendment rights have been violated, said David Cole, the ACLU’s national legal director.

“The courts have recognized that when government officials threaten punishment or consequences because of protected speech, that in and of itself can chill the speech, in violation of the First Amendment,” Cole said, citing a 1986 case in which a federal court sided with a challenge by Playboy Enterprises against Edwin Meese, then the U.S. attorney general, for sending letters threatening to publish a list of ­7-Eleven convenience stores that sold pornography.

It could be difficult, however, to establish a strong enough link between Trump’s threat and the NFL’s decision to show that the league was responding to pressure from the president.

Arizona Cardinals fans yell as San Francisco 49ers players kneel prior to an NFL game in Glendale, Ariz. (Ross D. Franklin/AP)

The aggrieved player would have to establish that he has standing to sue the government and that he was disciplined as a direct result of Trump’s actions. The NFL could argue that it came to the decision on its own — that it was not reacting to Trump, but to its audience melting away.

The NFL did not respond to a request from The Post for comment on Trump’s influence, but the league has said that Trump’s threat is misguided because the NFL relinquished its nonprofit status in 2015.

The White House later said that Trump was referring to public subsidies for sports stadiums in his tweet about the NFL's "massive tax breaks." Federal tax breaks for stadiums go to the teams. It is unclear how Trump could influence the tax-exempt bonds used to build stadiums. While the president oversees the Internal Revenue Service, he does not have the power to change tax law.

NFL Commissioner Roger Goodell said last fall that he supported players who want to see change in society, even if he didn’t necessarily agree with the protest.

“Players have a platform, and it’s his right to do that,” Goodell said at the time, in reference to Colin ­Kaepernick, the former San Francisco 49ers quarterback who kicked off the silent protests to highlight inequities in the criminal justice system.

Just weeks ago, some team owners locked arms with players after Trump called for the firing of “son of a bitch” players who protested. Several issued statements supporting the players’ freedom to express themselves and chastising Trump for his comments.

Dallas Cowboys owner Jerry Jones even knelt with his players just before the anthem. But he has since threatened to bench those who “take a knee,” drawing public praise from Trump. Jones said on his radio show that he decided to draw a “bright line” in part because of Trump’s activism in the debate. (Jones donated $1 million to Trump’s inaugural fund.)

Miami Dolphins owner Stephen Ross, who previously supported players for kneeling to protest racial inequality, told the Palm Beach Post that Trump "has changed that whole paradigm of what protest is." He said shifting public perception now makes it "incumbent upon the players today" to stand and salute the flag.

Hours after Trump’s tweet referring to tax breaks, Goodell sent a letter to the 32 team owners asking that they support a plan to ensure that players stand during the anthem “to honor our flag and our country.”

The NFL Players Association, along with player leadership, will attend this week’s league meetings to work with team owners on a possible resolution. The association did not respond to requests for comment about possible legal actions if the owners force players to stand.

The union could challenge a ban on anthem protests as a possible violation of its collective bargaining agreement, according to one person on the players’ side who spoke on the condition of anonymity because the owners have not reached a decision. Trump’s pressure tactics also might pave the way for a First Amendment challenge, the person said.

In a conference call with reporters Friday, Joe Lockhart, an NFL spokesman, did not respond to a question about whether the league believes team owners would be on sound legal footing if they require players to stand. Lockhart said the NFL does not yet have a firm proposal to require players to stand. “It’s something we’ve been working on with the players now for months,” he said.

Tennessee Titans wide receiver Rishard Matthews tweeted Thursday that he would quit playing football rather than be forced to stand during the anthem. The tweet, which was in response to a question posed by a TV producer, was quickly deleted. Matthews, whose father is a Marine and whose brother died in Afghanistan in 2015, has stayed off the field during the anthem.

Trump’s tweets calling for boycotts and firings alone are not a direct threat to workers’ free speech rights, legal experts say. But because he is president, a tweetstorm criticizing a company carries more weight and could harm a business’s bottom line.

“The fact this is happening strengthens the argument that a private entity’s action following something Trump suggests should be deemed state action and consequently could trigger constitutional protections,” said N. Jeremi Duru, a professor of sports law at American University.

Some constitutional scholars, though, remain skeptical that Trump’s threats could give players a First Amendment claim against the government.

“It takes more than a mere threat for a presidential statement to become state action,” said Alan Dershowitz of Harvard Law School. “I can imagine a case where the president’s statements constitute a deterrent, but it would be a real stretch.”

Dershowitz said to go down this path would pose a dangerous precedent for freedom of expression. “It would place constraints on the ability of the president to have a bully pulpit, whether he were a Democrat or a Republican.”

NFL players are contract workers who are also covered by a collective bargaining agreement that does not explicitly dictate that players must stand during the anthem. But the contract does stipulate that players could be terminated for engaging in “personal conduct reasonably judged by Club to adversely affect or reflect on Club.”

“The NFL players have sacrificed a lot of their First Amendment rights, given the broad language the players agreed to,” said Brad Snyder, a Georgetown Law professor who teaches constitutional law and sports law.

A local union filed a complaint last week with the National Labor Relations Board over Jones’s anthem edict for the Cowboys. The United Labor Unions Local 100 claimed that Jones violated federal labor law by threatening players’ jobs.

While Trump may provide players an opening to a First Amendment challenge if they are disciplined by the NFL for protesting, some First Amendment experts caution that a winning scenario remains unlikely.

“The government cannot pressure a business into engaging in patriotic speech,” said Eugene Volokh, a free speech scholar at the UCLA School of Law. “But it seems pretty clear that whatever action the NFL is going to take is going to be because of the very real risk of public reaction due to the drop off in enthusiasm for the NFL.”

The controversy over player protests comes at a time when NFL television ratings have dropped precipitously for a variety of reasons, including negative narratives about concussions and domestic violence.

Trump, during last year’s presidential campaign, directly blamed the low ratings on Kaepernick. The quarterback is now a free agent and has not been signed by another team. On Sunday, Kaepernick’s attorney, Mark Geragos, confirmed that he has filed a grievance accusing NFL teams of colluding to keep Kaepernick out of the league.

"Principled and peaceful political protest — which the owners themselves made great theater imitating weeks ago — should not be punished and athletes should not be denied employment based on partisan political provocation by the Executive Branch of our government," Geragos said in a statement.

Goodell’s move to curb the protests and tamp down anger by a Republican president and Republican-dominated Congress are motivated by profits, Snyder said. The NFL has a direct interest in protecting the antitrust exemption, granted through the 1961 Sports Broadcasting Act, that allows the league to pool broadcasting rights.

“Without that, the NFL could not survive, period. National television revenue is its life blood,” Snyder said. “There are huge stakes here, and none of it has to do with the national anthem.”

Mark Maske and Eli Rosenberg contributed to this report.