Fifteen days after President Trump was sworn in for the second time, and just a month into her first term in Congress, Representative LaMonica McIver of New Jersey donned a red blazer and took center stage at a large demonstration in Washington.

She and her Democratic colleagues had been denied entry into a U.S. Department of Treasury building after a team led by Elon Musk, Mr. Trump’s largest campaign donor, had been granted access to its payment system.

“I want Donald Trump to know that this Jersey girl means business,” Ms. McIver shouted into a microphone.

“Any time a person can pay $250 million into a campaign and then be given full access to the Department of Treasury of the United States of America,” she added, “we are at war.”

In far-right circles, the comments drew rebukes as an incendiary call to violence, and Ms. McIver, who is Black, was inundated by sexist and racist comments online.

Last week, Ms McIver again found herself at the center of controversy, as the Justice Department charged her with two counts of assault after a clash with immigration officers outside a Newark detention center that has become a flashpoint in Mr. Trump’s immigration crackdown.