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Sen. Rand Paul's neighbor appeared in a Kentucky state court Thursday and pleaded not guilty to a charge of fourth-degree misdemeanor assault.

Retired physician Rene Boucher reportedly blindsided Paul, tackling him from behind while the senator was wearing headphones and mowing his lawn. And while Boucher's motive remains a mystery, Paul's injuries offer clues as to the extent of the defendant's legal jeopardy: Six broken ribs and a pleural effusion, or build up of excess fluid around the lung — more serious than originally reported.

“Last week, Sen. Paul was vigorously assaulted by someone in his neighborhood. This is a serious criminal matter involving serious injury, and is being handled by local and federal authorities,” Doug Stafford, a senior adviser to Paul, told CNN Thursday.

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Paul’s injuries and his aide’s statement do not bode well for Boucher. Currently, he is charged with assault in the fourth degree, which is a misdemeanor. But that could change — so could the potential consequences if he is found guilty.

A defendant is guilty of assault in the fourth degree if he is found to have intentionally or wantonly caused physical injury to the victim. “Physical injury” is defined as substantial physical pain or impairment of physical condition, under Kentucky law.

The difference between this misdemeanor assault and felony assault in the second degree comes down to one word: “Serious."

A person commits felony second degree assault when he intentionally causes serious physical injury, according to Kentucky law, which is more than just physical injury. It includes prolonged impairment of health, or impairment of the function of any bodily organ, and broken ribs and impairment of the lungs appear to qualify. Ultimately, whether an injury is serious or just a misdemeanor injury is up to the jury at trial.

Paul’s newly-reported, more serious injuries could lead the state prosecutors to charge Boucher instead with the more-serious felony assault. The difference is punishment is massive. The misdemeanor assault carries a maximum term of a year of confinement, while a felony assault conviction carries a minimum of five and a maximum of ten years.

Could Boucher be charged with a federal crime? Ordinarily, an assault on an ordinary citizen does not involve federal law. But Paul is a U.S. senator, and federal law criminalizes assaults on members of Congress. Lesser assaults, like throwing a punch without landing the blow, or a slap that doesn’t injure, are punishable by one year of imprisonment and a fine.

Assaults that result in any personal injury are punishable by up to ten years of imprisonment.

There is no constitutional barrier to a federal assault prosecution of Boucher just because he’s being prosecuted in state court. This is because prosecutions by different sovereigns (the state and federal government) do not implicate the double jeopardy clause. But, a Department of Justice policy called the “Petite Policy” generally prohibits federal prosecution of a defendant following a state prosecution for the same act unless U.S. attorneys believe a federal crime was committed, and there is a substantial federal interest that the state prosecution failed to vindicate.

Federal prosecutors could easily conclude that a state prosecution for assault does not vindicate the different federal interest of protecting senators, and deterring attacks on public servants.

If prosecuted and convicted by both federal and state authorities, Boucher could face two different maximum sentences of ten years.