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The Roundtable was able to track how many people it had attracted, what they were willing to do on the issue's behalf and where they resided by Zip code and, therefore, by congressional district. Voila! The organization could target with real constituents almost any member of the House or Senate.

What's more, the Roundtable could see which ads were working by examining which were instigating the most (and the least) traffic to its Web sites. Only the ads that attracted the most clicks continued to run. The others were turned off, which saved money.

For the dividend tax fight of 2003, the process produced more than 500,000 advocates. There were so many, in fact, that the Roundtable was able to separate and then put to work senior citizens -- the most sympathetic type of person from the legislators' perspective -- in districts with representatives whose votes was up for grabs.

For the trade battle, the Roundtable recruited 170,000 grass-roots supporters. During the first half of 2005, they sent 1,500 e-mails to 240 members of Congress and 50 letters to editors of newspapers -- all in favor of approving the Central American Free Trade Agreement.

Those numbers are impressive and, in many respects, frightening. Major corporations were able to drum up broad backing on relatively obscure topics in a hurry. And then they won the day.

But a closer look indicates that this Internet revolution might have its limits. The call to arms on something as popular as a tax cut (the dividend proposal) produced a huge outpouring. Trade, a more abstract topic, barely tipped the scales.

The Roundtable had to augment its Web-based effort with far more costly and labor-intensive tactics. Through OnPoint, it also turned to traditional "grass-tops" lobbying. That entailed locating and then recruiting community leaders rather than hoi polloi in key districts to reach legislators from the home front.

For a reason that is as-yet undiscerned, the banner ads attracted constituents primarily from the East and West coasts and not many from the country's midsection, which was where the Roundtable needed them most.

"The Internet can be a very useful tool both to identify and to motivate people to express support to members of Congress," Castellani said. "But the more complicated the issue, the less valuable it becomes.

"It was very effective for the dividend issue, but trade didn't translate as easily. It was harder to make the direct connection to their economic wellbeing."

Castellani said he will continue to use the banner-ad method and considers it a powerful tool. But, he said, "it does not replace direct contact." Lobbying will always be lobbying."

From now on, though, advertising for advocates over the Internet will often be part of the mix.

Jeffrey Birnbaum writes about the intersection of government and business every other Monday. E-mail him atkstreetconfidential@washpost.com.