Thomas, 64, was first elected to Congress in 1978 and never pulled less than 59 percent of the votes in races for his California district. However, Republican term limits would have cost him his Ways and Means chairmanship next year and some colleagues predicted Thomas would not like to be without the powerful position.
At a news conference Monday in California, Thomas said he was leaving Congress because the cross-country trips were becoming a burden and he was still at his peak.
Thomas invoked former Los Angeles pitcher Sandy Koufax, saying "He was one of my favorites on the Dodgers, and I admired him when he said, 'I could pitch another game, but why?'"
The Washington Post said Thomas was one of the drafters of both the Medicare drug prescription plan and President George Bush's tax cut plans.
Rep. Rahm Emanuel, D-Ill., told the Post Thomas' decision is a sign Republicans are "sensing the winds of change" and support for change in Washington.
Political experts, however, told the Post Thomas' 22nd District is expected to remain in Republican hands.