Miller also became active politically with the
Democratic Party, but joined the brief offshoot known as the "
Union Democratic Party" in the political tumult following the
Mexican Cession. He was the Union Democratic nominee for
Wisconsin Senate in what was then the
4th Senate district, but was defeated by
John A. Eastman, the
Regular Democrat or "
Hunker" candidate. Miller ultimately returned to the Democratic Party after the
Compromise of 1850. In 1852, he was nominated for
Wisconsin State Assembly in Winnebago County's 1st district, which then comprised the southern half of the county, plus Oshkosh. He won the seat and served in the
6th Wisconsin Legislature. During that legislative term, he advocated for allowing the
Menominee people to remain in Wisconsin and furthered his plans for Fox River improvements. Also during that term, Miller was appointed by the governor to serve as chairman of the state board of commissioners for public works. He also became invested in the
Chicago and North Western Railway, and was elected to the board of directors. Miller sought the Democratic nomination for
Governor of Wisconsin at the 1859 Democratic state convention, but received only 15 of 206 delegate votes in the first round, then fell to just 7 votes in the second round, before withdrawing from the race. The following year, he ran for
Wisconsin Senate in the
21st Senate district, but lost the general election to
Horace O. Crane. After the start of the
American Civil War, Miller sometimes participated politically with the
National Union movement. He sought the Union nomination for
state bank comptroller in 1861, but was again defeated at the convention. Later that year, the Democratic convention's first choice for
State Treasurer of Wisconsin,
Hercules L. Dousman, declined the nomination. They subsequently offered the nomination to Miller, and he accepted. Dousman's name, however, still appeared on several Democratic tickets, and received votes. He went on to lose the general election to the Union nominee, incumbent
Samuel D. Hastings. After the 1861 election, Miller was out of politics for nearly a decade, focusing on his business interests. In 1871, he returned to elected office, running for
Winnebago County board of supervisors that spring. After winning that seat, he chose to run for state Assembly again, in Winnebago County's 1st Assembly district. He lost the fall election to Republican
Thomas Duncan Grimmer. He won re-election to the county board in 1872 and 1873, and was then chosen chairman of the county board in November 1873. Miller remained on the county board nearly continuously until his election to Congress in 1890. He was elected chairman of the board again in 1878, and was then re-elected each term as chairman for the rest of his years on the board. ==Congress==