LAS VEGAS — The day after a Democratic presidential debate brought out the candidates’ outrage at Michael R. Bloomberg, the focus turned back toward Senator Bernie Sanders of Vermont, who leads polls of Nevada, California and the nation.

Former Vice President Joseph R. Biden Jr. spent Thursday hammering Mr. Sanders for past positions that were friendly to the gun industry. Mr. Bloomberg warned that nominating Mr. Sanders would be “a fatal error.” And former Mayor Pete Buttigieg of South Bend, Ind., said he found it hard to imagine that Mr. Sanders could defeat President Trump in a general election.

“It would be a very, very tough sell,” Mr. Buttigieg told a crowd of students at a televised event at the University of Southern California. “I think it would be a tough sell with Mike Bloomberg, too. I’m saying we don’t have to choose between these two options. I’m saying there is another way.”

The attacks came two days before Nevada’s caucuses, the third contest of the Democratic presidential nominating calendar, and the day after a bitter debate that highlighted the dire situation that Mr. Biden and Mr. Buttigieg face. With Mr. Sanders favored to win in Nevada, they have limited time to try to stop his rise before the delegate-rich Super Tuesday contests on March 3 — when Mr. Bloomberg and his $300 million advertising campaign await.