“That said, my goal in life is to promote progressive values and policies,” he added. “How I accomplish that goal is always changing, and it will keep changing in large part based on the opportunities before me.”

Mr. Moulitsas is a prominent figure in progressive circles and would, as one of his associates noted, represent the same brand of pugnacious liberalism as embodied by Senator Elizabeth Warren of Masschusetts.

But he may not immediately have a chance to run for Ms. Lee’s seat. After The San Francisco Chronicle’s Matier & Ross column reported Sunday that Ms. Lee, a nine-term Democrat, was telling friends that she had reached a “gentlewoman’s agreement” with President Obama to be named ambassador to Cuba, now that relations are being normalized, Ms. Lee issued a statement this week saying that she would not “seek the nomination” to serve as the country’s envoy in Havana.