The barbs came as early voting for Nevada’s Democratic caucuses began on Saturday, with most of the leading candidates rallying supporters amid lingering concerns about whether Nevada would be able to avoid a repeat of the caucus fiasco in Iowa.

As Mr. Sanders addressed a crowd in a high school cafeteria on Saturday morning, with a mariachi band taking the stage before he did, his supporters were sharply focused on the process as well as on Mr. Sanders and his opponents.

Numerous people expressed concern about the logistics of a caucus in which they want to deliver a speedy and unequivocal victory to their candidate.

“I went to a training last week, and there are lots of concerns among the campaign — ‘How’s it actually going to work?’” said Paul Kleemann, a precinct captain volunteer for the Sanders campaign who teaches high school guitar in Las Vegas. “We were supposed to use the same app as Iowa, but they scrapped that, and as of Wednesday, they hadn’t released how they were going to get the early caucus results to the precincts.” He added, “It’s a little frightening at this point.”

In Nevada, early voting is being held from Saturday through Tuesday in advance of next Saturday’s caucuses. At an early caucus site at the East Las Vegas Library, voters waited in a 40-minute line on Saturday morning. After presenting their IDs, they filled in paper ballots by order of their choices, up to a total of five, then deposited them in a slotted box. The state Democratic Party said that almost 12,000 people had participated in the early voting as of 5 p.m. Saturday.

The ballots will be shared with the caucus precincts corresponding to their residences. If their first choice is not viable in their home precinct, the next in line will be counted if that person reaches the 15 percent threshold.

“This is the first time we’ve done early voting, and we’ve got to get this right because all eyes are on Nevada,” Representative Dina Titus, Democrat of Nevada, said on Friday. “And if we do a good job, then maybe we get to be first instead of Iowa next time around.”