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.

As a result, the legislative battle over two once-obscure bills to combat the piracy of American movies, music, books and writing on the World Wide Web may prove to be a turning point for the way business is done in Washington. It represented a moment when the new economy rose up against the old.

“I think it is an important moment in the Capitol,” said Representative Zoe Lofgren, Democrat of California and an important opponent of the legislation. “Too often, legislation is about competing business interests. This is way beyond that. This is individual citizens rising up.”

It appeared by Wednesday evening that Congress would follow Bank of America, Netflix and Verizon as the latest institution to change course in the face of a netizen revolt.

Legislation that just weeks ago had overwhelming bipartisan support and had provoked little scrutiny generated a grass-roots coalition on the left and the right. Wikipedia made its English-language content unavailable, replaced with a warning: “Right now, the U.S. Congress is considering legislation that could fatally damage the free and open Internet.” Visitors to Reddit found the site offline in protest. Google’s home page was scarred by a black swatch that covered the search engine’s label.

Phone calls and e-mail messages poured in to Congressional offices against the Stop Online Piracy Act in the House and the Protect I.P. Act in the Senate. One by one, prominent backers of the bills dropped off.

First, Senator Marco Rubio of Florida, a rising Republican star, took to Facebook, one of the vehicles for promoting opposition, to renounce a bill he had co-sponsored. Senator John Cornyn of Texas, who leads the G.O.P.’s Senate campaign efforts, used Facebook to urge his colleagues to slow the bill down. Senator Jim DeMint, Republican of South Carolina and a Tea Party favorite, announced his opposition on Twitter, which was already boiling over with anti-#SOPA and #PIPA fever.

Then trickle turned to flood — adding Senators Mark Kirk of Illinois and Roy Blunt of Missouri, and Representatives Lee Terry of Nebraska and Ben Quayle of Arizona. At least 10 senators and nearly twice that many House members announced their opposition.

“Thanks for all the calls, e-mails, and tweets. I will be opposing #SOPA and #PIPA,” Senator Jeff Merkley, Democrat of Oregon, wrote in a Twitter message. Late Wednesday, Senator Charles E. Grassley of Iowa, the senior Republican on the Senate Judiciary Committee, withdrew his support for a bill he helped write.

The existing bill “needs more due diligence, analysis and substantial changes,” he said in a statement.

Few lawmakers even now question the need to combat pirates at Web sites in China, Russia and elsewhere who have offered free American movies, television shows, music and books almost as soon as they are released. Heavyweights like the Walt Disney Company secured the support of senators and representatives before the Web companies were even aware the legislation existed.

“A lot of people are pitching this as Hollywood versus Google. It’s so much more than that,” said Maura Corbett, spokeswoman for NetCoalition, which represents Google, Amazon.com, Yahoo, eBay and other Web companies. “I would love to say we’re so fabulous, we’re just that good, but we’re not. The Internet responded the way only the Internet could.”

For the more traditional media industry, the moment was menacing. Supporters of the legislation accused the Web companies of willfully lying about the legislation’s flaws, stirring fear to protect ill-gotten profits from illegal Web sites.

Mr. Dodd said Internet companies might well change Washington, but not necessarily for the better with their ability to spread their message globally, without regulation or fact-checking.

“It’s a new day,” he added. “Brace yourselves.”

Citing two longtime liberal champions of the First Amendment, Senator Patrick Leahy and Representative John Conyers Jr. of Michigan, Mr. Dodd fumed, “No one can seriously believe Pat Leahy and John Conyers can be backing legislation to block free speech or break the Internet.”

For at least four years, Hollywood studios, recording industry and major publishing houses have pressed Congress to act against offshore Web sites that have been giving away U.S. movies, music and books as fast as the artists can make them. Few lawmakers would deny the threat posed by piracy to industries that have long been powerful symbols of American culture and have become engines of the export economy. The Motion Picture Association of America says its industry brings back more export income than aerospace, automobiles or agriculture, and that piracy costs the country as many as 100,000 jobs.

The House response, SOPA, was drafted by a conservative Republican, Representative Lamar Smith of Texas, with the backing of 30 co-sponsors, from Representative Debbie Wasserman Schultz of Florida, the chairwoman of the Democratic National Committee, to mainline Republican Peter King of New York. The Senate’s version, written by Mr. Leahy, the Vermont Democrat who is chairman of the Senate Judiciary Committee chairman, attracted 40 co-sponsors from across the political spectrum and cleared his committee unanimously.

Then the Web rose up. Activists said the legislation would censor the Web, force search engines to play policemen for a law they hate and cripple innovation in one of the most vibrant sectors of the American economy.

Mr. Smith, the House Republican author, said opposition Web sites were spreading “fear rather than fact.”

“When the opposition is based upon misinformation, I have confidence in the facts and confidence that the facts will ultimately prevail,” Mr. Smith said.

Google, Facebook and Twitter have political muscle of their own, with in-house lobbying shops and trade associations just like traditional media’s. Facebook has hired the former Clinton White House press secretary Joe Lockhart. Google’s Washington operations are headed by Pablo Chavez, a former counsel to Senator John McCain, Republican of Arizona, and a veteran of the Senate Commerce Committee.

And for all the campaign contributions, Washington parties and high-priced lobbyists the old economy could muster, nothing could compare to the tentacles the new economy can reach into Americans’ everyday lives through sites like Wikipedia. Aides to Senator Harry Reid, the majority leader, say he will press forward with a vote Tuesday to open debate on the Protect I.P. bill. Negotiators from both parties are scrambling for new language that could assuage the concerns of the Internet community, but expectations are that the bill will now fail to get the 60 votes to move forward — a significant setback.

“The problem for the content industry is they just don’t know how to mobilize people,” said John P. Feehery, a former House Republican leadership aide who previously worked at the motion picture association. “They have a small group of content makers, a few unions, whereas the Internet world, the social media world especially, can reach people in ways we never dreamed of before.”