Senator from New York to succeed William H. Seward who did not seek re-election, but would be appointed U.S. Secretary of State by Abraham Lincoln.

Although he supported the administration in the main, he did not fear to express his opposition to all measures, however popular at the time, that did not appear to him either wise or just.

His son William Hamilton Harris (1838-1895) was a brevet lieutenant colonel in the United States Army Ordnance Department.

His daughter Clara Harris and his stepson/future son-in-law Henry Rathbone were the Lincolns' guests at Ford's Theatre on April 14, 1865, when the president was shot and killed by John Wilkes Booth.

In the Senate, he also served on the Joint Committee on Reconstruction which drafted the Fourteenth Amendment to the United States Constitution.