Sen. Bernie Sanders (I-Vt.), addresses donors Thursday in Washington. It was only his third time soliciting funds in-person since launching his presidential campaign. (Aaron Davis/The Washington Post)

As a crush of millennials crowded into a brewery near Nationals Park on Thursday night, a young man setting up a loudspeaker for Sen. Bernie Sanders took the microphone. “Testing. One. Two. Three. The political revolution is here.”

The septuagenarian socialist who is disrupting the Democratic presidential race soon arrived and refined the message. The revolution is still coming, Sanders (I-Vt.) said. Hopefully on July 29.

On that Wednesday night, six months before the Iowa caucus, Sanders will livestream his case for the presidency to more than 1,500 simultaneous gatherings planned in bars, coffee shops and living rooms nationwide. The candidate’s address will be followed by an organizational meeting for anyone who wants to stay online and discuss joining his campaign.

In only his third fundraiser since announcing his candidacy in April, Sanders on Thursday night cast his campaign to a room full of 20-somethings in D.C. as a proxy for registering deep dissatisfaction with the state of American politics and the nation’s growing income inequality.

Sanders addresses the crowd at his fundraiser Thursday in Washington. (Aaron Davis/The Washington Post)

“Our job is to ask why,” Sanders said. “Why are we living in a society in which for the last 40 years the middle class in this country has been disappearing and almost all of the wealth and income are going to the people on top?”

Win or lose, Sanders said, “that’s the question we have to ask, that’s what this campaign is about.”

Whether Sanders can use the Internet to build an effective campaign remains to be seen, and the effort will not come without stiff competition. Hillary Rodham Clinton has hired a Google executive to lead her digital campaign, and in many ways she is building on the extraordinary success of President Obama’s online organization.

But there are signs Sanders has a base online to work from. In the first 24 hours of his campaign, 35,000 people donated an average of less than $44 apiece through his campaign Web site. After less than three months, he raised a total of $15 million, mostly from online donors who gave less than $200.

And the bulk of expenses for Sanders campaign so far have been to build up his online presence. He’s spent $1.3 million on digital consulting and online advertisements.

Sanders aides say that it’s all been building up to an event like the one on July 29 — something the Vermont Independent has long dreamed of.

Sanders has more than once paced his Senate office muttering to staffers that the only way a long-shot candidate like himself could reach enough people to be taken seriously would be to “beam himself into every living room” through the Internet, said Kenneth Pennington, a Senate aide who is now Sanders’s digital director.

For a candidate who has called for a $1 trillion public works plan and eliminating college tuition, he cast his campaign’s digital ambitions in no smaller terms, either. He said July 29 could be the country’s biggest online political event and his message could transcend political parties

“What we are trying, as part of creating a political revolution, is creating a grass-roots movement of millions and millions of people,” Sanders said. “On July 29 of this month, we will be holding what we believe will be the largest digital organizing event in the history of this country.

“We hope to have tens of thousands of people coming together to determine how they can develop movements in their local community,” he said.

“This campaign is not simply about electing me, I hope we accomplish that, but that ain’t the most important thing,” Sanders said. “The most important thing is building a political movement in which millions of people who have given up on the political process, including a lot of young people, get involved?”