He was elected to the Tennessee House of Representatives as a Democrat in 1976, replacing Larry Bates, who mounted an unsuccessful primary challenge against U.S.

When Jones retired in 1988, Tanner won the Democratic nomination for the seat and handily defeated Republican nominee Ed Bryant, who went on to represent the neighboring 7th District from 1995 to 2003.

As a Congressman, Tanner sponsored a bill to repeal the inheritance tax (which was vetoed by President Clinton) and he is in favor of a balanced budget.

Tanner was a founder of the Blue Dog Democrats and has denied rumors that he might switch parties, and has an earned reputation as a moderate.

Tanner was one of the few Democrats in the House to vote in favor of CAFTA and has long distanced himself from the majority of his party on issues such as bankruptcy law and lawsuit reform.

Tanner voted with the majority of his party to expand stem cell research and against renewing the controversial portions of the Patriot Act.

He drafted a bill that would require special bipartisan commissions rather than state legislatures to redraw congressional districts when necessary due to U.S. Census count changes.

It is generally believed that this bill was a response to Republican-inspired mid-decade redistricting in Texas and recent similar efforts in Colorado and Georgia.

After both his district and state chose the former first lady,[4] Congressman Tanner endorsed Senator Hillary Clinton's presidential campaign in April 2008.