Upon returning to Oregon, Thayer establuished a sucessful law firm in the city of East Portland. At this time, he became an active member of the Democratic Party. During the 1876 Hayes-Tilden Presidential Election dispute, Thayer was a member of the legal team which challened the certification of J. W. Watts, a Republican elector for Rutherford B. Hayes. Although sucessful, this challenge did not help Samuel Tilden prevail in the Electoral College.