The rejection was a remarkable rebuke, suggesting that ordinary citizens did not believe that the lawmakers had committed any crimes.

Federal prosecutors in Washington sought and failed on Tuesday to secure an indictment against six Democratic lawmakers who posted a video this fall that enraged President Trump by reminding active-duty members of the military and intelligence community that they were obligated to refuse illegal orders, four people familiar with the matter said.

It was remarkable that the U.S. attorney’s office in Washington — led by Jeanine Pirro, a longtime ally of Mr. Trump’s — authorized prosecutors to go into a grand jury and ask for an indictment of the six members of Congress, all of whom had served in the military or the nation’s spy agencies.

But it was even more remarkable that a group of ordinary citizens sitting on the grand jury in Federal District Court in Washington forcefully rejected Mr. Trump’s bid to label their expression of dissent as a criminal act warranting prosecution.

The move to charge the lawmakers — among them, Senator Mark Kelly of Arizona and Senator Elissa Slotkin of Michigan — was, by any measure, an extraordinary attempt by Trump appointees to politicize the criminal justice system even for a Justice Department that has repeatedly shattered norms of independence from the White House and followed Mr. Trump’s directives to prosecute his adversaries.

But manipulating bureaucratic levers is not the same thing as controlling the entire criminal justice system.

Before Mr. Trump’s second term, it had been exceedingly rare for grand jurors to rebuff requests by prosecutors seeking indictments. It is now happening with increased frequency, as Mr. Trump’s appointees push ahead with questionable cases in an effort to appease him.

On Tuesday, prosecutors presenting the case sought to persuade the grand jurors that the lawmakers had violated a statute that forbids interfering with the loyalty, morale or discipline of the U.S. armed forces, according to one of the people familiar with the matter.

They sought to bring charges against Mr. Kelly, Ms. Slotkin and their four colleagues in the House: Jason Crow, a former Army Ranger; Maggie Goodlander, a former Navy reservist; Chrissy Houlahan, a former Air Force officer; and Chris Deluzio, a Navy veteran.

The Justice Department declined to comment.

Ms. Slotkin welcomed the outcome. “No matter what President Trump and Pirro continue to do with this case, tonight we can score one for the Constitution, our freedom of speech, and the rule of law,” she said in a statement.

Mr. Kelly denounced the administration’s repeated efforts to target him and his colleagues.

“This is an outrageous abuse of power by Donald Trump and his lackeys,” he said in a statement. “Donald Trump wants every American to be too scared to speak out against him. The most patriotic thing any of us can do is not back down.”

Time and again, the Justice Department under Mr. Trump has brought questionable criminal cases against foes of the president — among them, James B. Comey, the former F.B.I. director, and Letitia James, New York’s attorney general.

Even though many of these cases have been weak, the department has apparently determined that it may be better to fail in court rather than push back against Mr. Trump’s well-known desire for revenge.

The president, emboldened by his success in bending the Justice Department and F.B.I. to his will, has intensified his effort to deploy the vast arsenal of federal law enforcement to pursue his political agenda and personal grievances.

The department recently opened investigations into Democratic officials in Minnesota who opposed his immigration crackdown, and arrested the journalist Don Lemon over his presence at a church protest in Minneapolis. Last week, the F.B.I. searched an elections office in the Atlanta area based on debunked claims that the 2020 presidential election was stolen.

In November, Ms. Pirro, a longtime Trump ally, approved a criminal investigation into Jerome H. Powell, the Fed chairman, whom Mr. Trump has long sought to supplant.

The case against the lawmakers was prompted by an online video organized by Ms. Slotkin, a former C.I.A. analyst who served multiple tours in Iraq. It did not mention any specific order or military scenario. But it was released as Mr. Trump was authorizing strikes against suspected drug boats in the Caribbean and considered deploying active-duty military troops to American cities to quell protests.

The lawmakers took turns reading a statement in which they cautioned that the “threats to our Constitution aren’t just coming from abroad, but from right here at home.”

“Our laws are clear,” said Mr. Kelly, a Navy veteran and former astronaut. “You can refuse illegal orders.”

Almost immediately, the video drew the ire of Mr. Trump, who demanded that the lawmakers be punished and even suggested that they should be executed.

“SEDITIOUS BEHAVIOR, punishable by DEATH!” Mr. Trump wrote on his social media site. He shared another person’s post that said, “HANG THEM GEORGE WASHINGTON WOULD !!”

Days later, the six lawmakers disclosed that the F.B.I. had contacted the House and Senate sergeants-at-arms requesting interviews with them, indicating that a criminal investigation was underway.

“President Trump is using the F.B.I. as a tool to intimidate and harass members of Congress,” the four House members who took part in the video said in a joint statement. “No amount of intimidation or harassment will ever stop us from doing our jobs and honoring our Constitution.”

Mr. Kelly is also facing a separate investigation by the Pentagon into what military officials described as “serious allegations of misconduct.”

In a social media post in November, Defense Secretary Pete Hegseth said that the video was “despicable, reckless, and false,” and he asserted that the lawmakers, whom he disparaged as the “Seditious Six,” were encouraging troops to “ignore the orders of their Commanders.”

“Their foolish screed sows doubt and confusion — which only puts our warriors in danger,” Mr. Hegseth wrote.