Dawn crept into Daniela Perdomo’s Brooklyn apartment, her bleary eyes still focused on her bright computer screen. The founder of a nearly three-year-old communications start-up, she had been up all night again, orchestrating the work of more than 100 other developers, web designers, researchers and writers.

But they were not working for Ms. Perdomo’s company. They were not even being paid. Instead, the all-night sessions were devoted to the Aug. 12 unveiling of FeeltheBern.org — a website they created to explain Bernie Sanders’s positions on issues in the 2016 presidential campaign.

Throughout the summer, a largely unseen legion of technology professionals across the country — software developers and designers, product managers and more — has been moonlighting online on Mr. Sanders’s behalf. The participants have been applying their initiative, creativity, organizational skills and, above all, technical prowess in their off hours to support an underdog candidate who has been more than grateful for the help.

They have built interactive maps of campaign events and are working on an automated flier generator to spit out polished promotional materials with a few clicks. They have set up an app to let volunteers and staff members at Mr. Sanders’s huge rallies use iPads to collect donations by swiping supporters’ credit cards. They have pioneered a tool to pair volunteers with lists of voters to call, automating what otherwise requires many paid campaign workers. And they have built dozens of websites for supporters of Mr. Sanders, the independent senator from Vermont, like Ms. Perdomo’s.