Representative Sheila Jackson Lee, a Democrat from Texas who was a leading voice for racial justice and progressive causes during the three decades she served in the House, died on Friday. She was 74.

Her death was announced in a statement from her family that did not list a cause of death. She said in June that she had been diagnosed with pancreatic cancer.

“By God’s grace, I will be back at full strength soon,” she told constituents at the time.

Ms. Jackson Lee, a former member of the Houston City Council who was elected to Congress in 1994, was an irrepressible presence from the start, relentlessly lobbying senior members for speaking time and almost always getting her way.

To critics, she would say that she was just serving her constituents.

“You have an obligation to make sure that their concerns are heard, are answered,” she said in a 1999 interview with The New York Times. “I need to make a difference. I don’t have wealth to write a check. But maybe I can be a voice arguing consistently for change.”

During her congressional career, Ms. Jackson Lee served as chairwoman of the Judiciary Subcommittee for Crime, Terrorism and Homeland Security, and as a senior member of the Judiciary, Homeland Security and Budget committees.

She was the author and lead sponsor of the legislation that in 2021 established Juneteenth, commemorating the end of slavery in the United States, as the first new federal holiday in 38 years.