casselman's immediate and avid interest in all branches of politics upon his arrival in prescott in 1919 led him to befriend politician j. d. reid, a prescott native who at the time had been mp for the grenville riding for nearly three decades. when reid retired from that office in 1921 after being appointed to the senate, it cleared the way for casselman's first federal election. in 1921 casselman was elected as mp for the first time in the grenville riding. the following year he resigned this seat to rt. hon. arthur meighen, a fellow conservative leader, to allow meighen to run successfully in an upcoming by-election.