GRAND RAPIDS, Mich. — A man convicted in a plot to abduct Michigan’s Democratic governor from her vacation home was sentenced on Tuesday to 16 years in prison, the longest sentence yet for a federal defendant in one of the country’s most closely watched domestic terrorism cases but far less than the life term that prosecutors sought.

At two trials earlier this year, prosecutors repeatedly showed recordings and online posts in which the defendant, Adam Fox, called Gov. Gretchen Whitmer a “tyrant,” railed against her Covid-19 restrictions and mused about a second American revolution. Prosecutors described him as a threat to the governor’s safety and to democracy itself.

“This is incredibly serious activity and there’s no doubt about that in my mind,” Judge Robert J. Jonker of the Federal District Court for Western Michigan said as he handed down the sentence at the courthouse in downtown Grand Rapids. But he said a life sentence was not necessary for Mr. Fox, who had no prior criminal record.

Federal prosecutors said Mr. Fox, who lived in the basement of a vacuum shop where he had worked, was a leader of the sprawling plot to take Ms. Whitmer from her vacation home in northern Michigan in 2020. Mr. Fox, an adherent of the so-called Boogaloo movement, which seeks to overthrow the American government, attended training sessions with heavy weaponry and went on scouting missions to that home in the months before his arrest.

“The conspirators might easily have killed the governor in a botched kidnapping, killed unsuspecting law enforcement during a traffic stop or other unexpected encounter, or blown up innocent bystanders with a negligently constructed bomb,” federal prosecutors said in a sentencing memo.

At trial, Mr. Fox’s lawyer, Christopher Gibbons, described his client as an unimpressive dupe who talked a big game but had no real chance of pulling off an attack. Mr. Gibbons said undercover F.B.I. agents preyed on Mr. Fox, pretending to be his friend and luring him into a plot he was incapable of planning himself. In arguing for a lesser sentence, Mr. Gibbons wrote that prosecutors used “exaggerated language to create the false narrative of a terrifying paramilitary leader.”

“Adam Fox was an unemployed vacuum repairman who was venting his frustrations on social media but abiding by the laws of the State of Michigan,” Mr. Gibbons wrote in a sentencing memo. He indicated that his client would appeal his conviction.

In court on Tuesday, Nils Kessler, a federal prosecutor, described Mr. Fox as a leader of the plot — “the essential man who really made everything happen.” He said that the kidnapping conspiracy could cause qualified people to rethink whether it was worthwhile or safe to serve in public office and that a stiff sentence was necessary to deter others who might consider similar plots.

“This is kind of a canary in a coal mine,” said Mr. Kessler, who cited the attack on the U.S. Capitol last year, which took place after the arrests in Michigan. “We have seen this again and again with plots that unfolded after this, including what happened Jan. 6.”

In explaining his decision not to issue a life sentence, Judge Jonker said Mr. Fox did not seem to be a natural leader, that his tactical skills were limited and that the plot had little chance of success because law enforcement had infiltrated the group.

Ms. Whitmer, who was elected to a second term last month, said after the trial that the convictions showed “that violence and threats have no place in our politics and those who seek to divide us will be held accountable.” Her spokesman did not immediately respond to a request for comment on Tuesday. The state’s attorney general, Dana Nessel, a Democrat, said in a statement that “Adam Fox’s actions undermined the security of every Michigan resident” and that his “sentencing sends a clear message that domestic terrorism will not be tolerated.”

At his first trial in the spring, jurors acquitted two of Mr. Fox’s co-defendants but failed to reach verdicts on the charges against Mr. Fox and another man, Barry Croft. At a second trial this summer, which played out amid a tense campaign season in politically divided Michigan, Mr. Fox and Mr. Croft were each convicted of kidnapping conspiracy and conspiracy to use a weapon of mass destruction.

Two other men, Ty Garbin and Kaleb Franks, pleaded guilty to kidnapping conspiracy in federal court and testified against Mr. Fox and Mr. Croft, who is set to be sentenced on Wednesday. Mr. Garbin was sentenced to 30 months in prison and Mr. Franks was sentenced to four years.

Three others connected to the plot were convicted in state court in October of providing support for terrorist acts. They received sentences that could keep them in prison at least seven years and up to 20 years. Five more men were charged in state court in another county and were awaiting trial.

Mr. Fox told Judge Jonker that he did not want to address the court before the sentence was announced. While prosecutors made their case for a life sentence, Mr. Fox, who was wearing an orange jail jumpsuit with a long-sleeved white undershirt, repeatedly looked toward the courtroom gallery.

When prosecutors announced charges against the men just weeks before the 2020 presidential election, the case quickly became one of the highest-profile domestic terrorism prosecutions in recent history. Many saw the plot as indicative of the rising threat of political violence and right-wing domestic terrorism, concerns that have only grown more pronounced since the riot at the U.S. Capitol on Jan. 6, 2021.

“Today’s sentence reflects the Department of Justice’s unwavering commitment to protecting our elected officials, law enforcement officers and dedicated public servants from criminal threats and violence — and to holding the perpetrators of such acts fully accountable under the law,” Matt Olsen, the assistant attorney general for national security, said in a statement on Tuesday.

But the prosecution had challenges, and securing convictions meant persuading jurors to trust a sprawling F.B.I. investigation that included several undercover federal operatives.

One F.B.I. agent on the case was fired last year after being charged with domestic violence, and another agent, who supervised a key informant, tried to build a private security consulting firm based in part on some of his work for the F.B.I., according to a BuzzFeed News report. Jurors did not hear the details of those incidents.

Judge Jonker, who was nominated to his position by former President George W. Bush, applauded the work of federal law enforcement when he handed down the sentence. He said the F.B.I. deserved “a pat on the back” for acting quickly, and that he saw no support for claims that the defendants were entrapped.

“They were there early,” Judge Jonker said of law enforcement, “and we should be thankful they were there early.”