“He’s a big talker,” Mr. Gibbons said of his client, whom he said lived in the basement of a vacuum store and had few friends. “He never really stops talking. He talks about things the government doesn’t like.”

But prosecutors have leaned heavily on recorded statements and social media posts to convince jurors that the plot was real.

“Which Governor is going to end up dragged off and hung for treason first?” Mr. Croft posted on Facebook in May 2020, according to an F.B.I. agent who examined his account.

Another F.B.I. agent testified that Mr. Caserta posted that January about the Second Amendment giving people the right to “kill agents of the government when they become tyrannical.” In May, near the height of the pandemic lockdowns, that F.B.I. agent said Mr. Caserta posted that it was “about time Americans use” the Second Amendment “for its intended purpose.”

Prosecutors said Mr. Croft, who lived in Delaware, repeatedly traveled to the Midwest for weapons training and meetings during the summer of 2020, including a nighttime “recon” mission to the governor’s vacation home near Elk Rapids, Mich., a rural town about 250 miles northwest of Detroit. Several of the men ended up driving aimlessly down her street because they wrote down the wrong address for the house.

On that same trip, an undercover F.B.I. agent who offered to sell the men explosives said he had joined Mr. Fox to scout out the bridge that, according to prosecutors, the men hoped to destroy to prevent the police from reaching Ms. Whitmer’s home after their planned abduction. When the agent spotted another bridge nearby, he testified that Mr. Fox had said they should blow it up, too.

Joshua Blanchard, a lawyer for Mr. Croft, accused the government of egging on his client, manufacturing a plot and attempting to use offensive social media posts as a stand-in for an actual conspiracy.

“They want to push this forward; they want to make it look like there was an agreement,” Mr. Blanchard said, downplaying his client’s words as the ramblings of “a truck driver who likes to get stoned and talk about the Constitution.”

Some defense lawyers emphasized that prosecution witnesses had offered differing accounts about how a boat, or more than one boat, might be used in the abduction. Prosecutors said the haziness about the details should not be confused with the absence of a conspiracy.

“They might have managed to abduct her from the house and figure out that this whole boat thing isn’t going to work,” Mr. Kessler said. He added: “Does that make them more or less likely to kill her?”

All four defendants face kidnapping conspiracy charges. Mr. Croft, Mr. Fox and Mr. Harris are also accused of plotting to blow up the bridge and are charged with conspiracy to use a weapon of mass destruction.

Federal and state prosecutors charged 14 men as part of their investigation. Two men pleaded guilty to a federal charge and testified before the jury hearing the federal case against the four men. Eight other men were charged in state court.

The disagreement at the heart of the federal case, about when rhetoric becomes a criminal conspiracy, was on display Thursday when Mr. Harris, 24, a Marine Corps veteran, took the witness stand. He was the only defendant to testify on his own behalf during the trial.

Mr. Harris’s lawyer repeatedly asked him whether he had agreed to kidnap the governor. Over and over, Mr. Harris said no.

But when prosecutors confronted him with chat messages about shooting Ms. Whitmer, he acknowledged sending them, including one in which he suggested posing as a pizza delivery driver and killing her when she answered the door.

During a tense cross-examination on Thursday, Mr. Harris seemed in his own way to distinguish speech from actions. Mr. Harris repeatedly used crude language to describe Dan Chappel, a military veteran who contacted law enforcement and became an F.B.I. informant after saying that he had become concerned about social media posts by militia members. Mr. Chappel pretended to befriend Mr. Harris and the other men and at times helped to organize training outings for the group, all while surreptitiously recording their conversations and reporting back to agents.

“Words hurt you? Words scare you?” Mr. Harris said under cross-examination, referring to Mr. Chappel, who had testified earlier in the trial that he feared the group’s anti-government and anti-law enforcement rhetoric would escalate into violence.

Mr. Harris said he considered Mr. Chappel to be the group’s leader.

Several of the men who were charged had been members of the Wolverine Watchmen, a militia group that held training outings and participated in Second Amendment protests, including some at the Michigan Capitol. But prosecutors and defense lawyers have described a blend of overlapping anti-government ideologies among the defendants, including Three Percenter, boogaloo movement and anarchist affiliations.

Mr. Harris testified Thursday that he had long been concerned about an eventual breakdown in civil order. He said that some of his messages displayed in court were about actions he would take after such a breakdown happened, not about a particular plot to kidnap the governor, as prosecutors have suggested. Mr. Harris said he did not consider Ms. Whitmer to be a tyrant but thought she was bad at her job.

Throughout the trial, prosecutors displayed a trove of high-powered rifles and body armor that they said would have been used in the attack on Ms. Whitmer. They also exhibited Hawaiian shirts, which are frequently worn by boogaloo adherents, and the anarchist flag that the F.B.I. seized from the apartment of Mr. Caserta.

In response, defense lawyers have suggested that the prosecution is criminalizing the exercise of First and Second Amendment rights. “The anarchy flag — there’s nothing illegal about hanging political flags, is there?” Michael Hills, an attorney for Mr. Caserta, asked an F.B.I. agent.