In all, he backs at least $18 trillion in new spending over a decade, according to a tally by The Wall Street Journal, a sum that alarms conservatives and gives even many Democrats pause. Mr. Sanders sees the money as going to essential government services at a time of increasing strain on the middle class.

His agenda includes an estimated $15 trillion for a government-run health-care program that covers every American, plus large sums to rebuild roads and bridges, expand Social Security and make tuition free at public colleges.

To pay for it, Mr. Sanders, a Vermont independent running for the Democratic nomination, has so far detailed tax increases that could bring in as much as $6.5 trillion over 10 years, according to his staff.