US President Donald Trump has called for the National Football League (NFL) to be stripped of tax advantages.
In a tweet he complained the league had received “massive” tax breaks, while some of its players had been “disrespecting” the national anthem.
The NFL league office, which overseas the rules, referees and the college draft, is registered as not-for-profit, tax exempt organisation.
Teams pay tax, but have often received public support to build stadiums.
Mr Trump has criticised players sharply for the protests and pressed the NFL to ban them.
On Tuesday, US Vice-President Mike Pence walked out of an NFL game after several players refused to stand.
Mr Pence said he could not be present at an event that “disrespects our soldiers, our flag” after abandoning the game in his home state of Indiana.
President Donald Trump tweeted that he had asked Mr Pence to leave if players kneeled and said he was “proud of him”.
Former San Francisco 49ers quarterback Colin Kaepernick started the protest when he sat down as the Star-Spangled Banner played during a pre-season game in 2016.
He explained his gesture, which later became known as taking a knee, was meant to highlight racial injustice and police brutality against African Americans.
Angered by Mr Trump’s comments on the issue, dozens of other players have joined the protest.