President Donald Trump has ratcheted up the national controversy over black National Football League players who refuse to stand while the U.S. National Anthem is played before games.

During a Friday night political rally in Alabama, Trump called on fans to boycott teams that allow players to engage in that particular form of protest.

The league's TV ratings have slid since quarterback Colin Kaepernick, then the leader of the San Francisco 49ers, began the trend in September 2016.

'Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, "Get that son of a b***h off the field right now! He is fired. He's fired!"' Trump boomed.

His crowd applauded and chanted 'USA! USA!'

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President Donald Trump told an arena full of Alabama supporters Friday night that NFL fans should walk out of stadiums when players refuse to stand during the playing of America's National Anthem

The trend was started a year ago by quarterback Colin Kaepernick (center) who was then playing for the San Francisco 49ers

Trump said Friday that NFL owners would be instantly popular if they pulled protesting players off the field, and that fans should vote with their feet

The outspoken president also said American football is 'not the same game anymore' since referees have clamped down on aggressive tackles, throwing penalty flags whenever they hear the crack of a vicious hit.

'They are ruining the game, right?' he asked. 'They are ruining the game.'

Players on the field 'want to hit,' he complained.

'But you know what's hurting the game more than that?' Trump asked. 'When people like yourselves turn on television and you see those people taking the knee when they are 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.'

'Pick up and leave. Just pick up and leave.'

Trump said the NFL's ratings slide is in part because of player protests. The half-empty stadium at Thursday's San Francisco-Los Angeles game is shown

The President was in northern Alabama to stump for Luther Strange, a Republican who is running in a special election for the U.S. Senate seat he was appointed to when former Sen. Jeff Sessions became attorney general.

Kaepernick has been unable to find a team that will sign him this year after a season of highly visible anti-racism protests

In a sometimes rambling 80-minute stemwinder, Trump said it's only a matter of time before the owner of an NFL team decides to bench – or cut entirely – a player who refuses to join in the weekly show of patriotism.

'You know, some owner is going to do that. He is going to say, "That guy that disrespects our flag, he's fired",' he predicted.

'That owner, they don't know it – they are friends of mine – they don't know, they are going to be the most popular person, for a week. They will be the most popular person in this country.

'Because that's a total disrespect of our heritage, that's a total disrespect of everything that we stand for.'

The president acknowledged that kneeling during 'The Star Spangled Banner' is a legitimate form of First Amendment-protected protest, but still said fans should vote with their feet.

'I know that we have freedoms,' he said, 'and we have freedom of choice. And many, many different freedoms.'

This week the trend reached its logical conclusion when a suburban St. Louis youth football team made up of eight-year-olds knelt during the playing of the National Anthem

Trump was in northern Alabama to stump for Sen. Luther Strange, who is running in a special election for the seat he was appointed to when Jeff Sessions became attorney general

Kaepernick said last year that his protest, which inspired similar acts of defiance on several teams, was in response to a history of racism in the United States.

'I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color,' he told reporters following a preseason game last season.

'To me, this is bigger than football and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.'

Kaepernick, who at one time was San Francisco's pass-throwing wunderkind, was an injured backup player when he began his protests.

A year later, he has found himself unable to find a team willing to sign him.