“On issue after issue, there should be no doubt in anybody’s mind as to whether Hillary Clinton is the superior candidate because in every respect she is,” Mr. Sanders said to the crowd of about 300 people. “I intend, as a United States senator, to do everything that I can between now and Election Day to make certain that Donald Trump does not become the next president of the United States.”

But Lois Corcoran, a former Sanders supporter who plans to vote for Ms. Stein, said she refused to accept the idea that people must vote for Mrs. Clinton if they do not want to see Mr. Trump in office. She said it would not be her fault if the businessman is elected and is frustrated that Mr. Sanders will not back Ms. Stein, who she believes shares more of his ideas.

“I feel very betrayed by him,” said Ms. Corcoran, 52, of Charlestown, N.H. “He says it’s not the man, it’s the message, but yet he’s jumped over to the Democrats’ message.”

In his speech, Mr. Sanders, sensing that some of his supporters were still weighing whom to support, stressed that no candidate was perfect and that the election was not about personalities but about the “needs of the American people.”