WASHINGTON (Reuters) – U.S. President Donald Trump ramped up with his fight with the National Football League on Tuesday, calling on the popular league to ban players from kneeling in protest at games while the U.S. national anthem is played.

“The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can’t kneel during our National Anthem!” Trump wrote on Twitter, fuelling his war of words with the multibillion-dollar NFL in his fifth straight day of public comments on the issue.

Representatives for the league and its players union could not be reached immediately for comment.

Trump earlier Tuesday praised two NFL teams that had largely steered clear of the controversy Monday night. The Arizona Cardinals linked arms and stood for the playing of the “Star-Spangled Banner” along with the Dallas Cowboys, who knelt before the song.

“But while Dallas dropped to its knees as a team, they all stood up for our National Anthem. Big progress being made-we all love our country!” Trump wrote, adding that “ratings for NFL football are way down.”

Television networks reported a mixed impact on viewership for Sunday’s games.

It was the latest salvo from Trump, a former reality television show host and political neophyte who took office in January, after he ignited the fight with the players in the biggest-grossing U.S. pro sports league last week.

On Friday, he told a political rally that any protesting player was a “son of a b****” who should be fired, and urged a boycott of NFL games, touching off protests by dozens of players, coaches and some owners before games on Sunday.

Trump’s verbal assault may play well with his conservative base at a time when the Republican president is grappling with North Korea’s nuclear threats, a humanitarian crisis in hurricane-struck Puerto Rico, an investigation into Russian meddling in the 2016 election and a healthcare struggle in Congress.

Hillary Clinton, Trump’s ex-rival in the 2016 presidential contest, blasted Trump for targeting black players and stoking racial tensions.

“He’s very strategic about who he attacks, and he is sending a message. It’s a huge loud dog whistle to his supporters,” she said in an interview on the “CBS This Morning” program.

Former San Francisco 49ers quarterback Colin Kaepernick first kneeled during the national anthem last year, refusing to stand to protest police shootings of unarmed African-Americans.

Several players have made similar gestures in what they said is a call for social justice, not a slight against the country or its flag.

Critics, including Trump, have said it is disrespectful. Supporters have said the protests embody the American right to free speech.

(Reporting by Susan Heavey, Jonathan Allen and Makini Brice; Editing by Jeffrey Benkoe)