When Judge Aileen M. Cannon assumed control of the case stemming from former President Donald J. Trump’s indictment for putting national security secrets at risk, she set the stage for the trial to be held with a regional jury pool made up mostly of counties that Mr. Trump won handily in his two previous campaigns.
She signaled that the trial would take place in the federal courthouse where she normally sits, in Fort Pierce, at the northern end of the Southern District of Florida. The region that feeds potential jurors to that courthouse is made up of one swing county and four others that are ruby red in their political leanings and that Mr. Trump won by substantial margins in both 2016 and 2020.
She left open the possibility that the trial could be moved — and political leanings are not necessarily indicative of how a jury will decide — but the fact that the trial is expected to draw jurors who live in places that tilt Republican has caught the attention of Mr. Trump’s allies and veterans of Florida courts.
“For years, it’s been a very conservative venue for plaintiffs’ lawyers,” said John Morgan, a trial lawyer who founded a large personal injury firm. Describing the various counties that feed into Fort Pierce, he said, “It is solid, solid Trump country.”
In Okeechobee County, a rural county where just over 16,000 people voted in the 2020 election, Mr. Trump won 71.5 percent of the vote, according to the county’s election tally. In Highlands County, a rural area where more than 52,000 people voted in that election, Mr. Trump won with 66.8 percent of the vote.
In Martin County, where more than 98,000 people voted, Mr. Trump got 61.8 percent of the vote. In Indian River County, which contains Vero Beach and where more than 97,000 votes were cast, Mr. Trump got 60.2 percent of the vote.
Only St. Lucie County, where about 172,000 votes were cast, is a swing district. Mr. Trump eked out a victory there over President Biden in 2020 with 50.4 percent of the ballots cast, the data shows, and also won the county narrowly in 2016.
Dave Aronberg, an outgoing Florida state attorney in Palm Beach County, said he could recall few major or politically sensitive cases in the Fort Pierce courthouse. He agreed that the Fort Pierce counties provide a “much more conservative jury pool,” although he suggested that a number of prospective jurors could be drawn from St. Lucie, which is more politically diverse.
Judge Cannon, who was appointed by Mr. Trump in 2020, disclosed in an order on Tuesday that the trial and all the hearings connected to it would likely be held in Fort Pierce, about 120 miles north of Miami along the east coast of Florida.
She left open the possibility of eventually moving the trial, noting in her order that “modifications” could “be made as necessary as this matter proceeds.”
The trial of a former president who is also the front-runner for the 2024 Republican nomination is likely to involve substantial security issues as well as logistical challenges given the crush of interest in the case.
When Mr. Trump was arraigned this month, the proceeding took place at the large federal courthouse complex in Miami, likely because the duty magistrate assigned to the initial hearing was based there. But now that Judge Cannon will handle the remainder of the case, it became her prerogative to move it to Fort Pierce, one of four other cities in the Southern District of Florida to have a federal courthouse. (Courthouses in Miami, Fort Lauderdale and West Palm Beach sit in counties that Mr. Biden won in 2020.)
The Fort Pierce courthouse, which sits on a busy state highway a few blocks from the water, is Judge Cannon’s home base. She is the sole district judge working from the building.
First the Justice Department and then the special counsel, Jack Smith, investigated Mr. Trump’s mishandling of classified documents for months in front of a grand jury in Washington. Had the case been prosecuted there, the former president and his allies would have almost certainly raised concerns about the fairness of the jury pool in the city.
Many rioters charged in connection with the Capitol attack on Jan. 6, 2021, sought to move their trials from Washington by claiming that local residents were largely liberals.
But not one of the numerous attempts to move the trials elsewhere was approved by a judge. And Mr. Trump’s advisers are well aware that Florida, which Mr. Trump carried twice, is a more beneficial place for this particular defendant.
Mr. Aronberg suggested that Judge Cannon’s order allowing flexibility could be a signal of a change down the road.
“I’m not convinced this case is going to go in Fort Pierce,” he said, predicting a potential move to West Palm Beach, which would put it in the county where Mr. Trump lives and where the classified documents in question were stored after he left office.
