A Harvard University professor argues that the NFL's decision to ban kneeling by players during the national anthem is illegal.

In an op-ed for Vox published Friday, Harvard labor and industry professor Benjamin Sachs writes that the NFL's decision this week to force players to remain in the locker room if they wish to protest during the anthem violates both labor laws and the First Amendment.

"The clearest illegality derives from the fact that the league adopted its new policy without bargaining with the players union," Sachs writes. Kneeling during the anthem, which Sachs says is a clear form of workplace protest, must be discussed with the NFL players' union before such a policy can be banned.

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"If, as the NFL Players Association says, the employer implemented this change on its own, the policy is flatly illegal for that reason and should be rescinded by the league," he added.

The other reason for the policy's illegality, Sachs argues, surrounds President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's involvement in the issue.

Both Trump and Vice President Pence have taken firm stands against players kneeling during the anthem to protest police brutality and racism. Trump has also publicly called on team owners to change league policy and fire players who participate in the protest.

"The president of the United States has been actively involved in the league’s decision-making process," Sachs wrote.

"In an earlier round of the protest dispute, President Trump called on the league to discipline Colin Kaepernick for his leadership of the anthem protests and threatened to use the tax code to punish the NFL if they allowed them to continue."

Sachs argued that this pressure from Trump and Pence brings into play the players' constitutional right to not have their speech censored by the government.

"When the president and vice president of the United States are this intimately involved in encouraging a private employer to adopt a workplace rule, the Constitution should have something to say."

NFL team owners announced this week that they had unanimously voted to "impose appropriate discipline" on players who participate in anthem protests going forward.

"We believe today's decision will keep our focus on the game and the extraordinary athletes who play it — and on our fans who enjoy it," NFL Commissioner Roger Goodell said Wednesday after all 32 team owners approved the decision.