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BOSTON — Bernie Sanders, eager to bounce back from his loss Saturday in the Nevada caucuses, pointedly attacked Hillary Clinton on Monday, arguing that she was copying his message and that she might be improperly influenced by Wall Street donations to her candidacy.

Though the Vermont senator has repeatedly said he wants to avoid a negative campaign, Mr. Sanders delivered one of his most striking critiques of Mrs. Clinton yet, accusing her of mirroring his style and highlighting their differences on the campaign finance system and trade policies. The comments come as Mr. Sanders is fighting to prove he can remain viable in the race, which will mean broadening his support beyond the thousands of young people and working-class whites that pack his events.

“I have to say that I am delighted that Secretary Clinton, month after month after month, seems to be adopting more and more of the positions that we have advocated. That’s good,” Mr. Sanders said, adding that Mrs. Clinton was “beginning to use a lot of the language and phraseology that we have used.”

“In fact, I think I saw a TV ad and thought it was me. But it turned out it was Secretary Clinton’s picture in the end,” Mr. Sanders said at a news conference at an ironworkers union office in Boston.

Mr. Sanders went on to imply that Mrs. Clinton’s stances on issues were not as genuine as his own.

“But the people of Massachusetts and the people of the United States need to know the difference between hastily adopted campaign rhetoric and the real record and long-held ideas of the candidates,” Mr. Sanders said.

Mr. Sanders has said that the Iowa caucuses, where Mrs. Clinton narrowly beat him, and the New Hampshire primary, where Mr. Sanders won by double digits, proves that his message of a rigged economy is resonating with voters. However, Mrs. Clinton’s victory in the Nevada caucuses on Saturday delivered a blow to Mr. Sanders’s momentum and raised questions about how much longer he can remain in the race.

The Democratic delegate count so far shows Mr. Sanders slipping significantly behind Mrs. Clinton.

Without directly accusing Mrs. Clinton of being influenced by Wall Street money, Mr. Sanders pointed out that Priorities USA Action, the “super PAC” that is supporting Mrs. Clinton’s presidential campaign, raised $25 million during the last reporting period from special interest groups, including $15 million from Wall Street.

“Now I know that every candidate who has ever received special interest money always says that the millions and millions of dollars they have received will never influence them. Never, never, never,” Mr. Sanders said. “Well, the American people need to ask a very simple question: If these contributions from Wall Street and other powerful special interests have no influence over the candidate, why are these special interests making huge campaign contributions?”

“Simple question. Now maybe they are dummies. And maybe they just think they throw millions of dollars and expect to get nothing. Maybe. I doubt that very much.”

On Tuesday morning, Mrs. Clinton’s campaign responded to Mr. Sanders’s attacks.

“How revolutionary: after a loss in Nevada and facing an uphill climb in South Carolina, Bernie Sanders is breaking his campaign pledge once again to trot out old, tired attacks on Hillary Clinton,” said Christina Reynolds, a spokeswoman for Mrs. Clinton’s campaign.