When the powerful world of old media mobilized to win passage of an online antipiracy bill, it marshaled the reliable giants of K Street — the United States Chamber of Commerce, the Recording Industry Association of America and, of course, the motion picture lobby, with its new chairman, former Senator Christopher J. Dodd, the Connecticut Democrat and an insider’s insider.

Yet on Wednesday this formidable old guard was forced to make way for the new as Web powerhouses backed by Internet activists rallied opposition to the legislation through Internet blackouts and cascading criticism, sending an unmistakable message to lawmakers grappling with new media issues: Don’t mess with the Internet.