A radical part of the Republican Party were incorrigous with the decisions taken by Lincoln during his term. They created the Radical Democratic Party, nominating John C. Fremont as a candidate. Which would take a lot of votes from Lincoln.
However, to prevent the Democrats from winning, Fremont resigned his candidacy so as not to detract from Lincoln's votes, and along with some important and unforeseen victories in the Civil War, Lincoln could obtain his reelection.