“Democrats have enjoyed watching Republicans fight among themselves for the last five years, but there are anti-establishment and ideological divisions in the Democratic Party that will grow larger in the years to come,” said Nathan L. Gonzales, editor of The Rothenberg & Gonzales Political Report.

The disaffection among Democrats flows mainly from three sources, according to interviews with voters and strategists. Disappointment lingers with President Obama over the failure to break up big banks after the Great Recession and fight for single-payer health insurance, among other liberal causes. Fatigue with Mrs. Clinton’s controversies endures, as does distaste with her connections to the rich. And anger abounds at party leaders for not pursuing an ideologically pure, economically populist agenda.

“Establishment Democrats like Hillary could end up heavily outspending people like Sanders, but it may not matter as much as usual because voters are searching for someone off the beaten path,” said Paul Maslin, who was the pollster for Howard Dean’s 2004 presidential race.

Tom Henderson, the Democratic chairman in Polk County, Iowa, which includes Des Moines, said the most widely shared frustration among Democrats there was “the manner in which the economy has bounced back” under Mr. Obama: Wall Street returns look strong and unemployment has declined, but wages and benefits are largely unchanged.

“I think Sanders is pulling in voters who aren’t typical rank-and-file Democrats, but rather folks who have become energized over the last few years to change the country’s direction,” said Mr. Henderson, who is currently neutral in the race. “The question for Sanders is whether he can get those people to show up and vote in February.”

Across the country, discontent on the left is bubbling up in scattered ways.

Liberal activists in Iowa and elsewhere have pushed local Democratic officials to enact minimum-wage ordinances in the face of inaction from state and federal government officials in both parties. Environmentalists put enormous pressure on Mrs. Clinton to come out against the Keystone pipeline project, which she did last month. Many progressives in New York City complain that the Democratic mayor, Bill de Blasio, appears to have lost his appetite for a criminal justice overhaul.