A Democrat, he served Webster and Calhoun county in the Iowa Senate from 1995 to 1999, representing the 7th District.

From 1972 to 1978, Halvorson worked for Iowa Congressman Berkley Bedell, serving as a campaign organizer and congressional district manager.

During this time, Halvorson served as a member of the Iowa Democratic State Central committee, from 1978 to 1982, 1992–94 and 2000 and as an assistant House majority leader for the 70th-72nd General Assemblies.

[4] In 2015 and 2016 Halvorson became actively involved in the Senator Bernie Sanders campaign for president and was elected as a Minnesota DFL national delegate to the Democratic National Convention and elected to serve as the Minnesota DFL delegation co-chair for Sanders.

[10][11] In 2014, Halvorson attempted a political comeback by returning to Fort Dodge and running for the Webster County Board of Supervisors in District 4.