Trump claimed on Twitter Friday that "most of them are unable to define" what they're demonstrating against.
Instead, Trump tweets players should "Be happy, be cool!"
Numerous player demonstrations took place during the national anthem at several early NFL preseason games Thursday night. Players have been protesting police killings of black men, social injustice and racism.
Trump writes from his New Jersey golf resort that players "make a fortune doing what they love" and that those who refuse to stand "proudly" for the anthem should be suspended without pay.
Trump has told associates he believes the anthem issue is a winning one that riles up his base.
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