Democratic officials say they are catching up. The Democratic National Committee hired Raffi Krikorian, a former Uber executive, as its chief technology officer last year. Dan Sena, the executive director of the Democratic Congressional Campaign Committee, said that the group’s digital budget had tripled, and that the party was “light-years ahead of where it was” in 2016 because of new investments in its digital capabilities. Democrats are also getting help from outside groups like Higher Ground Labs, which finances start-ups that build campaign technology.

Ms. Alter, 38, who sold her start-up last year and now works as an entrepreneur-in-residence at the venture capital firm Social Capital, came up with the idea for a tech volunteer network in early 2017, just after President Trump issued his travel ban targeting residents of several Muslim-majority countries.

Outraged about the policy, and looking for a way to help, she enlisted Peter Kazanjy and Ian Ferguson, two friends in tech who shared her liberal politics. They agreed that Silicon Valley tech workers would jump at the opportunity to apply their technical skills to campaigns. How different is selling a candidate online, after all, from selling shoes or shaving kits?

“This is not rocket science,” Mr. Kazanjy said. “Campaigns are online/offline e-commerce plays, where the transaction is a vote.”

They sent a Google form to their friends in the tech industry to gauge interest. Within three days, more than 700 people had signed up. The group has since grown to more than 4,500 volunteers, raised more than $100,000 in a crowdfunding campaign, and moved into an office in downtown San Francisco that it shares with a venture capital firm.

Nick Hobbs, 29, first heard about Tech for Campaigns while working at Google. Mr. Hobbs, who left the company last year, had always been politically active, but Tech for Campaigns gave him a way to channel his efforts into something productive.