Mr. Bouricius, who had a front-row seat to Mr. Sanders’s anguish, said issues like war and militarism were important to him. “But they were all seen as one step down from what he saw as the fundamental issues, which was capitalism and the privilege of wealth,” he said.

That clarity has been critical to Mr. Sanders’s aura of authenticity and has attracted millions of young voters. It has also provided a stark contrast with Mrs. Clinton, who has echoed him by saying “the economy is rigged in favor of those at the top” but who still has trouble convincing voters she is trustworthy.

In the 1970s, Mr. Sanders argued his message from the political fringe, and then, starting in 1981, from his perch as the Socialist mayor of Burlington. From that office he sent news releases like one that read, “If the working people and people of moderate income do not become increasingly involved in the contemporary political struggles then the political decisions will continue to reflect the interests of the small clique of money people who presently dominate state and national politics.”

The Burlington Free Press highlighted a quotation of Mr. Sanders’s from 1971 — “the real issue is who controls the wealth of this country” — and observed that 14 years later, “He’s still singing the same tune.”

In 1988, he ran for Congress, sending a note on personal stationery inviting Vermonters to help him “make basic changes in an economic and political system in which 1% of the population owns over 50% of the wealth.”

In countless speeches and interviews and debates in Congress, the Senate and now on the presidential trail, Mr. Sanders has essentially said the same thing over and over. Mr. Bouricius worried, though, that Mr. Sanders had not done enough to actually build the political revolution he had spent 40 years calling for. “If Bernie gets hit by a bus,” Mr. Bouricius said, "a lot of this just goes away.”