President Donald Trump on Friday fired another shot at NFL players who refuse to "stand proudly" for the national anthem.

Trump took to Twitter to mock the athletes who were "at it again," encouraging them to “Be happy, be cool!” since they make a “fortune doing what they love.”

The president’s tweets come hours after NFL players -- including Kenny Stills and Albert Wilson of the Miami Dolphins, Robert Quinn of the Miami Dolphins and Malcolm Jenkins of the Philadelphia Eagles -- protested during the anthem as the league began its first full week of the 2018 preseason.

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Lou Holtz: Players 'Hurting the Future' by Kneeling for National Anthem Before NFL Games

The NFL, in May, modified its national anthem policy, prohibiting any sort of demonstrations for the 2018 season and making individual teams responsible for disciplining players who protest on the field.

The players' union filed a grievance about the policy change, and late last month, the new policy was put on hold while the NFL and NFL Players Association work on a resolution.

On "Outnumbered," Katie Pavlich said it seems like the players who continue to protest during the anthem have no interest in coming to the middle or understanding why so many people find their demonstrations offensive.

She said that NFL owners clearly want the issue to go away because it's affecting their bottom line, but their collective bargaining agreement with the players association prevents them from taking unilateral action.

"It's because of the players union, it's because of the collective bargaining power that they have against the owners and the league and [NFL Commissioner Roger] Goodell," Pavlich said.

"They don't have any control over the disciplinary process, what the rules are. They've had rules that are broken and there's no consequences for the players. And it's because they have the collective bargaining power. That is the problem."

Watch more from "Outnumbered" above.

LaVar Ball to NFL Protesters: Stand for the National Anthem or 'Get Out'

Keith Ellison Boycotting NFL Over 'Cowardly & Idiotic' Anthem Policy

Trump on 'Fox & Friends': Maybe NFL Players Who Don't Stand for Anthem 'Shouldn't Be in the Country'

Artist Tackles National Anthem Controversy With Painting of Trump Holding Trampled American Flag