Boucher then executed a "running tackle" on Paul, 55, in the yard of his Bowling Green, Ky., home, according to court records. Paul, who had been mowing his lawn and was wearing headphones, did not see Boucher until the last second.
Boucher, who had previously been charged with a misdemeanor in a county court and had pleaded not guilty, denied in an interview with police that there was any political motivation for the assault, according to court documents. He said it was a property dispute that finally boiled over.
Boucher "continues to be very regretful and very remorseful," said Matthew J. Baker, an attorney for Boucher. "He's looking forward to gaining some closure on this."
Paul's office declined to comment.
Federal prosecutors will recommend a sentence of 21 months, according to court papers. No date has been set for a plea hearing. The U.S. attorney for the Western District of Kentucky was recused from handling the case, so it was transferred to Indiana.
"Assaulting a member of Congress is an offense we take very seriously," U.S. Attorney Josh J. Minkler said in a statement. "Those who choose to commit such an act will be held accountable."
The assault, in an upscale gated community, drew widespread media attention and generated a political mystery, since Paul and Boucher have personally offered few clues about what sparked one of the worst assaults on a sitting senator in decades.
In late November, Baker, Boucher's attorney, told The Washington Post that it was tied to simmering disagreements between the successful doctors about the maintenance — "or lack of it" — of their adjacent lots.
Baker said Boucher and Paul didn't see eye to eye over the care of grass, trees and other aspects of their properties.
Kelley Paul, the senator's wife, dismissed that explanation in a November op-ed for CNN, and Rand Paul said the motivation was beside the point in a Jan. 7 interview with "Face the Nation."
"I think one of the things about motivations is people got obsessed, some in the media, about the motivations. But I think, really, we usually don't ask if someone's raped or mugged or whatever why the person did it," he said.
Paul described the attack in a Fox News interview in December.
"I was working in my yard with my earmuffs on — you know, to protect my hearing from the mower — and I had gotten off the mower, facing downhill, and the attacker came running full blown," Paul said. "I never saw him, I never had conversation — in fact, the weird thing is, I haven't talked to him in 10 years."
Kentucky State Police said they were called to Paul's house around 3:20 p.m. on Nov. 3. A police report on the incident indicates that Boucher admitted going on Paul's property and tackling him.
Police said Paul initially refused medical care, thinking his rib injuries were not so severe. He was eventually treated after the extent of the injuries became apparent. Boucher was charged with fourth-degree assault in Warren County, Ky.
"He is profoundly regretful," Baker said of his client in November. "He wishes this had never happened."
Baker confirmed that Boucher hadn't talked with Paul in years, saying the pair had lapsed into a stony silence over the landscaping issues.
Boucher, a retired anesthesiologist, and Paul, an ophthalmologist, had lived next door to each other in the Rivergreen community for 17 years and once worked at the same hospital. Boucher's gabled house sits on a corner lot across an expanse of grass and trees from Paul's red-brick colonial.
Bill Goodwin, an acquaintance of both men, said he arrived at their properties about a decade ago as the men were finishing a dispute. Boucher told Goodwin that Paul had blown lawn clippings onto his yard and that it angered him.
"I ask him, I tell him, and he won't pay attention," Goodwin recalls Boucher saying after the argument. "One of these days."
Paul said on "Face the Nation" that his recovery has been arduous.
"It was sort of, I guess, a living hell for the first four or five weeks," Paul said. "Couldn't get out of bed without assistance, six broken ribs, damage to my lungs, two bouts of pneumonia. It was really a tough go of it. But each day I feel a little bit better. This last month I've been doing better."