The “right” candidate is particularly important in building coalitions for statewide races, the experts said. Progressive candidates like Abdul El-Sayed in Michigan and Cynthia Nixon in New York failed to draw support outside of urban centers. In Delaware, two days after Ms. Pressley’s victory, Kerri Harris had hoped to unseat an incumbent senator but secured only 35 percent of the vote.

Step 2: Take advice from Silicon Valley: Test. Iterate. Repeat.

A nimble, grass-roots campaign must also be responsive to voter feedback, the experts said. Whether it’s listening to the advice of on-the-ground organizers, or tailoring digital messages to response patterns, the advantage of a nontraditional campaign should be its ability to quickly adjust.

“The whole point of digital is you can test what messages are working,” said Jessica Alter, the co-founder of Tech for Campaigns, a group of more than 8,000 tech workers who are volunteering to use their digital skills to help elect progressive candidates in November.

Testing different messages on small groups of Facebook users before launching a large-scale ad campaign, for example, could reveal that voters in a candidate’s district prefer positive ads to negative ads. Or, it could identify a national issue — say, immigration reform or marijuana legalization — that stirs up passions among a candidate’s target voter groups and might be a good subject to include in a stump speech. Testing messages online can even work for old-fashioned print advertising.

“If you’re going to spend $20,000 on direct mail, maybe spend $700 and test it on Facebook in a targeted way first,” she said.

Newer tools aren’t an automatic path to victory, of course. And social media popularity doesn't always translate to votes. In New York, Ms. Nixon dominated the conversation on Twitter, but she was handily defeated by incumbent Governor Andrew Cuomo, whose campaign quietly spent more than $2 million on digital advertising, with a heavy focus on Facebook.

Ms. Pressley’s campaign convened a weekly feedback circle of community leaders, local activists and volunteers. Sarah Groh, Ms. Pressley's 29-year-old campaign manager, said the advisers allowed the team to notice where their opponent was making gains. When the group felt some of the incumbent’s messages were beginning to take hold in the later stages of the race, they decided that Ms. Pressley should record another video ad, focused on delivering her final message to voters in an easy-to-digest fashion.