The sharp exchanges in the Nevada debate reflected the urgency of the moment as Michael Bloomberg, in his first presidential debate, was repeatedly criticized in personal terms.

The Democratic presidential candidates turned on one another in scorching and personal terms in a debate on Wednesday night, with two of the leading candidates, Senator Bernie Sanders and Michael R. Bloomberg, forced onto the defensive repeatedly throughout the evening.

In his first appearance in a presidential debate, Mr. Bloomberg, the former mayor of New York City, struggled from the start to address his past support for stop-and-frisk policing and the allegations he has faced over the years of crude and disrespectful behavior toward women. Time and again, Mr. Bloomberg had obvious difficulty countering criticism that could threaten him in a Democratic Party that counts women and African-Americans among its most important constituencies.

Two candidates who have shied away from direct conflict in past debates, Senator Elizabeth Warren of Massachusetts and former Vice President Joseph R. Biden Jr., mounted something of a tag-team onslaught against Mr. Bloomberg, several times leaving him visibly irked and straining to respond.

From the first seconds, when Mr. Sanders used the initial question to attack what he called Mr. Bloomberg’s “outrageous” policing record, it was clear that this debate would be far more heated than any of the previous forums. The unrelenting attacks reflected the urgency of the moment, as Mr. Sanders gains strength and those hoping to slow his candidacy are increasingly crowded out by Mr. Bloomberg and his unprecedented spending spree.

Ms. Warren landed the most stinging blows against Mr. Bloomberg throughout the debate, starting with an opening broadside that likened him to the figure most reviled among Democrats: President Trump.