Bernie Sanders’ campaign has been marked by the huge crowds turning out to his rallies, packing arenas with 20,000 people or more. But the key to his staying power in the 2016 race lies in more modest venues — living rooms, libraries and Internet chat apps — where his campaign turns the senator's online fans into a volunteer army that surpasses anything seen in presidential politics.

Barack Obama's successful campaigns in 2008 and 2012, for all their digital accomplishments, never fully achieved their ambition of converting online support into a volunteer force that could do the grunt work of cold-calling voters or knocking on doors.

But Sanders’ team has — honing a strategy of turning "slacktivists" who don't normally engage in grass-roots politics into an advance team capable of doing everything from managing phone banks to planning high-level campaign events. As of early April, his tens of thousands of networked volunteers had made 47 million phone calls, putting them on track to surpass the calls made by Obama’s operation during the entire 2012 election cycle.

Sanders’ organizational success — fueled by free or low-cost, off-the-shelf apps like Hustle and Slack — is the lesser-known counterpart to his campaign’s prowess in raking in campaign cash from hordes of shallow-pocketed donors online. Experts in the evolution of technology in political campaigns say these innovations have helped Sanders put up a far more vigorous than expected challenge to Democratic front-runner Hillary Clinton, including his 13-point win in Tuesday’s Wisconsin primary and his 12-point victory Saturday in Wyoming.