“This system is very frustrating,” she said.

Some progressives were upset by the groundswell of late-breaking establishment support for Mr. Biden, saying it was a sign that party elites were again trying to deny Mr. Sanders the nomination, after his defeat in 2016 by Hillary Clinton.

“It’s probably going to backfire, just like it did with Hillary,” said Robin Killingsworth, a retired computer-support worker from Adams County, north of Denver, who was voting in his first primary on Tuesday. “The Democratic establishment seems to be screwing us around.”

Mr. Sanders’s strength with Latinos was expected to be a major factor in the Super Tuesday states of California, Texas and Colorado. At another voting site in Adams County, which is about 40 percent Hispanic, José and Mayela Garcia Alba pulled up in their rattling bronze sedan to vote for Mr. Sanders. The couple run a small housecleaning business and obtain their health coverage through Medicaid, and said they did not know when — or if — they would be able to afford to retire. They liked Mr. Sanders’s proposals to expand public health care and cut high prescription costs.

“The conditions now are primed for a person like Bernie,” said Mr. Garcia Alba, 62.

Texas has the second-largest number of delegates to award on Tuesday, after California, and Mr. Biden was hoping to keep it close there, especially after winning the endorsement on Monday of former Representative Beto O’Rourke, who ran a brief presidential campaign and nearly won a Senate seat in Texas in 2018. Four years ago, Mrs. Clinton defeated Mr. Sanders in the state among not only white and black voters but also Latinos.