Although the networks themselves do not hold federal licenses, their individual television stations do. His threat to target NBC drew immediate concerns that he was undermining the First Amendment.

“Broadcast licenses are a public trust,” said Tom Wheeler, who until January was chairman of the Federal Communications Commission, appointed by President Barack Obama. “They’re not a political toy, which is what he’s trying to do here.”

In suggesting that a broadcast network be targeted because of its coverage, Mr. Trump evoked the Watergate era, during which President Richard M. Nixon told advisers to make it difficult for The Washington Post to renew the F.C.C. license for a Florida television station it owned. A businessman with ties to Mr. Nixon filed paperwork to challenge The Post’s ownership of the station. The Justice Department under Mr. Nixon also filed antitrust charges against the three major television networks.

In Mr. Trump’s case, it may just be an idle threat, the sort of bluster that he has regularly used to keep perceived adversaries off balance. Just a day earlier, he suggested using federal tax law to punish the National Football League as part of his campaign against players who kneel during the national anthem, only to have a spokeswoman later say he was just making a point.

But Mr. Wheeler said it could also be taken as instruction to supporters who could act on his behalf. “This sounds to me like another dog whistle for folks to file against the license renewals,” he said. “Clearly it would be a bridge too far for the Trump F.C.C. to move on their own initiative. But if some conservative groups were to take this as their marching orders, it would be an interesting situation to see what the Trump F.C.C. did.”

Shortly after the tweets, Senator Edward J. Markey, a Democrat from Massachusetts, wrote a letter to Ajit Pai, the current F.C.C. chairman, urging him to protect First Amendment rights. “I ask for your commitment to resist the president’s request and call on you to publicly refuse to challenge the license of any broadcaster because the president dislikes its coverage,” Mr. Markey wrote.