A member of the Democratic Party, he defeated Republican incumbent Jeff Denham in the 2018 election by five points.

[2] He won reelection to California's new 9th congressional district, created through the 2020 redistricting process, which includes the majority of San Joaquin County.

[15][16] California's 10th district was included on the list of Republican-held seats being targeted by the Democratic Congressional Campaign Committee in 2018.

[23] Following redistricting, Harder defeated San Joaquin County Supervisor Tom Patti, a Republican, with 56% of the vote in California's 9th Congressional district.

After Trump supporters stormed the United States Capitol on January 6, 2021, Harder received hate mail intended for Josh Hawley, a United States senator with a similar name who objected to certifying Joe Biden's electoral college victory.

He voted for a police reform bill that would ban chokeholds and establish a policy for use of force that would be standard around the country.

[36] In 2023, he introduced a bill to create a grant program that would allow smaller police departments to recruit and retain more officers.