A federal judge has rejected President Donald Trump's free speech defence against a lawsuit accusing him of inciting violence against protesters at a campaign rally.

Trump's lawyers sought to dismiss the lawsuit by three protesters who say they were roughed up by his supporters at a March 1, 2016 rally in Louisville, Kentucky. They argued that Trump didn't intend for his supporters to use force.

Two women and a man say they were shoved and punched by audience members at Trump's command. Much of it was captured on video and widely broadcast during the campaign, showing Trump pointing at the protesters and repeating "get them out." Judge David J Hale in Louisville ruled on Friday that the suit against Trump, his campaign and three of his supporters can proceed.

Hale found ample facts supporting allegations that the protesters' injuries were a "direct and proximate result" of Trump's actions, and noted that the Supreme Court has ruled out constitutional protections for speech that incites violence. "It is plausible that Trump's direction to 'get 'em out of here' advocated the use of force," the judge wrote. "It was an order, an instruction, a command."

Plaintiffs Kashiya Nwanguma, Molly Shah and Henry Brousseau allege that they were physically attacked by several members of the audience, including Matthew Heimbach, Alvin Bamberger and an unnamed defendant they have yet to be able to identify.

Bamberger later apologised to the Korean War Veterans Association, whose uniform he wore at the rally. He wrote that he "physically pushed a young woman down the aisle toward the exit" after "Trump kept saying 'get them out, get them out," according to the lawsuit.

Heimbach, for his part, sought to dismiss the lawsuit's discussion of his association with a white nationalist group and of statements he made about how Trump could advance the group's interests. The judge declined, saying such information could be important context when determining punitive damages.

The judge also declined to remove allegations that Nwanguma, an African-American, was the victim of racial, ethnic and sexist slurs from the crowd at the rally.

This context may support the plaintiffs' claims of negligence and incitement by Trump and his campaign, the judge said. "While the words themselves are repulsive, they are relevant to show the atmosphere in which the alleged events occurred," Hale wrote.