In recent days, Mr. Sanders appeared to acknowledge the odds against him, and began speaking less about beating Mrs. Clinton and more about working to defeat Donald J. Trump, the presumptive Republican nominee.

On Sunday, he gathered with about 20 key supporters and advisers at his home in Burlington, Vt., to discuss how to proceed.

“We are going to take our campaign to the convention with the full understanding that we are very good at arithmetic and that we know, you know, who has the received the most votes up to now,” Mr. Sanders said after the meeting, standing on his front lawn with his wife, Jane. Among the dozen or so people who attended the gathering were Benjamin T. Jealous, a former president of the N.A.A.C.P.; Congressman Raúl M. Grijalva of Arizona; Nina Turner, a former Ohio state senator; and Bill McKibben, the environmentalist and author.

Notably, Mr. Sanders also said he would continue his efforts aimed at “transforming the Democratic Party,” a sign that his main goal may no longer be to become the nominee.