Mr. Waxman’s departure after 20 terms in the House will be particularly poignant. One of his most notable accomplishments, the Affordable Care Act, which he was instrumental in writing, is shaping up as the centerpiece of campaigns all over the country, not as a triumph but as a Republican cudgel. And the expansion of Medicaid that he has championed has been challenged in a number of states run by Republican governors.

The sprawling bill to combat climate change that he wrote was passed by the House in 2009 but died in the Senate, and President Obama has given up on efforts to push it through. Mr. Waxman has also spent years trying to strengthen the powers of the Food and Drug Administration and the Environmental Protection Agency, but those efforts are under fire from the Republicans who control the House.

Still, Mr. Waxman will leave behind a legacy of entrenched accomplishments, including the Children’s Health Insurance Program, which extends coverage to millions of low-income children; anti-tobacco, food safety and food-labeling laws; and the Ryan White Care Act, which allocates billions of dollars in federal money for the treatment of H.I.V. and AIDS.

He is also credited with laying the foundation for many of the executive actions that Mr. Obama, during his State of the Union address on Tuesday, pledged to pursue.

One involves the Clean Air Act, which Mr. Waxman helped write and which gives the Environmental Protection Agency the authority it is now exercising to regulate power plant emissions of greenhouse gases. Mr. Waxman saw to it that the bill would allow the president, on his own, to order improvements in automobile fuel efficiency and other energy saving efforts.