Representative Gerald E. Connolly of Virginia, an eight-term Democrat, announced on Monday that he would not seek re-election and would soon relinquish his position as the top Democrat on the House oversight committee, as he faces cancer.

Mr. Connolly, 75, announced late last year that he was being treated for cancer of the esophagus but planned to fight the disease while continuing to do his job in Washington, saying he was “very confident of a successful outcome.”

In a letter to his constituents on Monday, he said that the disease, “while initially beaten back, has now returned,” prompting his decision to step aside and ultimately retire. Mr. Connolly said he planned to do “everything possible” to finish out what he said would be his final term.

“I will be stepping back as ranking member of the Oversight Committee soon,” he wrote. “With no rancor and a full heart, I move into this final chapter full of pride in what we’ve accomplished together over 30 years.”

Mr. Connolly’s announcement did not make clear when he may hand over his position, and a spokesman did not immediately respond to a request for comment. The Committee on Oversight and Government Reform is scheduled to hold a crucial hearing on Wednesday about its portion of Republicans’ tax and spending program, which includes changes to federal employees’ benefits and gives them the option to give up civil service protections to make more money.

Mr. Connolly’s decision will clear the way for a new senior Democrat on one of the most partisan committees in Congress, which lawmakers typically use to antagonize the White House and exercise their check on executive power.

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