Who was a former union general that ran against Abraham Lincoln for president in 1864?

Former General George B. McClellan ran against Lincoln in 1864.

1864 Presidential Election The candidates for the Presidential election of 1864 were; Abraham Lincoln - Andrew Johnson (Republican), and George McClellan - G. H. Pendleton (Democratic). Abraham Lincoln won the election and was reelected into his second term. The 16th President of the United States Abraham Lincoln was in office from March 4, 1861 until April 15, 1865 when he was assassinated.