Sen. Bernie Sanders, whose liberal call to action has propelled his long-shot presidential campaign, is proposing an array of new programs that would amount to the largest peacetime expansion of government in modern American history.

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.

A campaign aide said additional tax proposals would be offered to offset the cost of some, and possibly all, of his health program. A Democratic proposal for such a “single-payer” health plan, now in Congress, would be funded in part through a new payroll tax on employers and workers, with the trade-off being that employers would no longer have to pay for or arrange their workers’ insurance.

Mr. Sanders declined a request for an interview. His campaign referred questions to Warren Gunnels, his policy director, who said the programs would address an array of problems. “Sen. Sanders’s agenda does cost money,” he said. “If you look at the problems that are out there, it’s very reasonable.”

Calling himself a democratic socialist, Mr. Sanders has long stood to the left of the Democratic Party, and at first he was dismissed as little more than a liberal gadfly to the party’s front-runner, Hillary Clinton. But he is ahead of or tied with the former secretary of state in the early-voting states of Iowa and New Hampshire, and he has gained in national polling. He stands as her most serious challenger for the Democratic nomination.

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