In 1855, Brainerd was one of the organizers of the new Republican Party, which was founded as the country's main anti-slavery political organization. He served as the first chairman of the Vermont Republican Party, and was one of five state party chairmen who issued the call for the first Republican National Convention in 1856. Brainerd was a delegate, and as the temporary chairman he called the convention to order for its first session. Brainerd was subsequently chosen to serve as one of the convention's vice chairmen, and was appointed as a member of the Republican National Committee. Brainerd was chosen as one of Vermont's presidential electors after the 1856 election. When Vermont's electors met in December to cast their ballots, Brainerd was chosen as their chairman. They all voted for the Republican ticket of John C. Frémont for president and William L. Dayton for vice president, who carried Vermont but lost the election.