It's hard to imagine two issues of less interest to most voters than free trade with Central America and a cut in the tax on dividends paid to the owners of stock.

Yet the latest in lobbyists' wiles -- a grand elaboration on Internet advertising -- managed to rope hundreds of thousands of average Americans into the congressional debate on both matters and was instrumental in passing the measures into law.

The until-now unheralded trick-of-trade was bankrolled by the Business Roundtable -- an organization of chief executives from 160 large companies -- and was executed by Alexandria-based Democracy Data & Communications LLC and its OnPoint Advocacy affiliate. It worked so well that the Roundtable is experimenting with it as a way to revive President Bush's foundering effort to make private accounts part of Social Security.

"It's the future of lobbying," said John J. Castellani, president of the Roundtable.

Why? Because the gimmick turns the greatest-ever tool of the masses -- the Internet -- into a gold mine for factions that seek lots of people to contact members of Congress on their behalf. When legislators hear from a bloc of constituents on a subject, they tend to believe that support for it is widespread, even when the communication is actually prompted (read: manufactured) by a well-financed and narrowly focused lobby.

The technique works this way: An interest group that wants to gather home-grown advocates takes out a banner advertisement on a widely used Web site. By clicking on the ad, people acknowledge that they agree with the group's opinion and are then asked what further steps they'd be willing to take to help the cause. These include writing letters to the editor and calling, writing or meeting with lawmakers in the capital or back in the district.

The interactive ads, in other words, create instant, ad hoc lobbying organizations that can be mobilized on every front that modern influencers utilize.

In several respects, this is a significant advancement over older methods of "grass-roots" lobbying. First, the would-be advocates are self-selected; they are already eager to press the case. In the past, Washington-based lobbyists had to hunt for grass-roots helpers by using expensive telemarketing and postal mailings. They often had to persuade people to adopt their arguments and train them how to talk to their elected representatives.

Second, the cost of collecting followers is much reduced. The Roundtable had to pay only about $1.50 per advocate for its dividend fight, compared $9 to $13 per advocate for patch-through calls to the offices of members of Congress gathered via telephone banks.

Third, the grass-rooters found through the Web can be used and reused as a lobbying base. "You can keep going back to them on similar issues for years to come," said Thomas M. Herrity, president of OnPoint.

The Roundtable advertised on financial Web sites frequented by people who were concerned about money, the economy and fiscal policy. Its ads asked readers to click on them if they believed in the point of view they advanced.

Example: An ad that purported to reveal a way to create 2.5 million new jobs said: "Click to eliminate the unfair, double tax on dividends & help get America back to work!" A reader who clicked was transported to a site that asked for a name, address and other particulars including whether the person was willing to apply pressure on his or her congressman.

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 at kstreetconfidential@washpost.com.

Through interactive Internet ads, the Business Roundtable involved thousands of Americans in congressional debate.John J. Castellani of the Business Roundtable praised the Web method.