7:11 PM PT -- Rand Paul suffered five broken ribs, 3 of which are either partly or completely cracked, from this weekend's attack ... according to his Chief of Staff Doug Stafford.

Rand Paul was allegedly "blindsided" at his home by his 59-year-old neighbor and cops have arrested the guy ... TMZ has learned.

The Kentucky Senator was assaulted Friday afternoon by Rene Boucher and suffered a minor injury ... according to law enforcement. Cops were called to the house and determined Boucher did it on purpose and arrested him. He was charged with one count of fourth-degree assault.

Boucher reportedly tackled Paul from behind at his Bowling Green residence after the 2 had an argument. He's being held on $5k bond at the Warren County Detention Center.