After checking the roof for bullet casings from the shooting, Monk finds one in a hidden slot. Leland identifies it and tells Randy to call Grooms at the bureau and to notify ATF. Agent Grooms appeared in S3:E12 "Mr. Monk Gets Cabin Fever (2005)". He was the Agent in Charge of keeping Adrian safe at the hideout cabin, along with Leland and Natalie.
Monk mentions during the campaign that Dr. Kroger is Jewish (which is why he doesn't celebrate Christmas). Stanley Kamel, who played Dr. Kroger, was Jewish in real life.
When Monk is retrieving the sign that Natalie's daughter Julie made, he is seen getting it from behind the boxes twice.
Mr. Monk never actually casts his ballot! He and Natalie rush off, leaving the ballot in the booth.