President Trump’s efforts to use the Justice Department to exact new punishments on people against whom he nurses old grudges appear to be ramping up.

Over the weekend, in a remarkably direct post on his social media website, Trump instructed Attorney General Pam Bondi to “move now” to prosecute his adversaries, defying a principle of independence that has characterized the relationship between the Justice Department and the White House going back to Watergate.

And today, one of his former personal lawyers, Lindsey Halligan, was sworn into a key prosecutorial role in Virginia. Days earlier, her predecessor indicated that he was unlikely to bring charges against one Trump foe — Attorney General Letitia James of New York — because of a lack of evidence. He also raised concerns about a potential case against the former F.B.I. director James Comey.

This is an important moment, one that could be easy to miss in the crush of news. So I called my colleague Devlin Barrett, who covers the Justice Department, and asked him to explain why it matters. Rank-and-file prosecutors, Devlin explained, are becoming increasingly concerned about pressure to bring indictments even when evidence seems flimsy. It’s all shaping up to become a bigger battle over what their department is really for.

It’s unusual for a president to order an attorney general to essentially prosecute his enemies. Why?

For basically two generations, there has been a generally understood principle that the president and his direct advisers simply do not direct criminal investigations, because that is a tool that could be used improperly to go after opponents and help political allies. That’s what the Nixon administration got in trouble for.