The election of 1864 was widely considered by many as a referendum on the Civil War. Were Lincoln to be re-elected, the Union campaign would continue uninterrupted. However, should Lincoln suffer a defeat, the Union's authority would be seriously undermined with its commander-in-chief toppled by an unsupportive electorate.
As the election approached, Lincoln's chances of success looked slim for several reasons. For one, with Grant entrenched outside of Petersburg and Sherman entrenched outside of Atlanta, the Union armies were making little progress at the expense of heavy casualties. In addition, Lincoln had to endure the embarrassment of a French-backed occupation of Mexico, when Napoleon III set up the Archduke Maximillian as Emperor in the spring of 1864 in a blatant violation of the Monroe Doctrine. However, with American forces engaged in the Civil War, Lincoln was helpless to strike back, and the French would hold Mexico until after the war's conclusion.
Struggling at home and faltering abroad, doubt swirled as to whether Lincoln would even be re-nominated by the Republicans. In February of 1864, Senator S. C. Pomeroy of Kansas had issued a circular recommending the nomination of Secretary of Treasury Chase over Lincoln. This breakaway factionalism led to the resignation of the increasingly truculent Chase from his cabinet post. However, Lincoln did not begrudge his rival, and appointed Chase as Chief Justice of the Supreme Court upon the death of Taney in December of the same year.
Amidst echoes of revolt, a convention of dissident Republicans met in Cleveland at the end of May, eventually shifting their support from Chase to General Fremont. Fremont, the initial Republican candidate of 1856, and a staunch abolitionist, had seen early successes during the Civil War in western theater, where his emancipation of slaves in Missouri had been repealed by Lincoln. The Radical Republicans, displeased with Lincoln's mysteriously effective combination of moderate politics and a dictatorial style, saw Fremont as a leader who, unlike Lincoln, would support a stern plan for reconstruction. However, a month before the election, when Fremont realized that running as a third candidate would likely hand a victory to the Democrats, he withdrew from the race. In so doing, he left Lincoln with a much clearer chance at re-election.
In a show of solidarity, the Republicans combined with the so-called War Democrats to form the National Union Party, which nominated Lincoln for president at a convention held in Baltimore on June 8. To balance the interests of this makeshift party, leading War Democrat and Acting Military Governor of Tennessee Andrew Johnson was nominated for vice president.
For their part, the Democrats convened on August 29 in Chicago, on Lincoln's home turf. To oppose him, they nominated General McClellan on a platform of negotiated peace. But one week after being nominated, McClellan accepted without embracing the party platform. In this manner, the Democrats hoped to broaden their electoral base, winning anti-war votes on the strength of their platform, and anti-Lincoln votes on the strength of McClellan's record.