Senator Bernie Sanders said on Sunday that he would “take our campaign for transforming the Democratic Party into the convention,” refusing to concede the presidential nomination to Hillary Clinton though not explicitly saying he would challenge her for it.  Mrs. Clinton earned enough delegates to clinch the nomination last week, but Mr. Sanders has declined to end his campaign. He has contended that he could persuade enough superdelegates, the party leaders who have overwhelmingly backed Mrs. Clinton, to switch their support to him by arguing that he would be the stronger candidate against Donald J. Trump, the presumptive Republican presidential nominee.  That plan became more improbable last week as high-profile Democrats supported Mrs. Clinton. President Obama endorsed her on Thursday, calling her the most qualified candidate ever to seek the White House and imploring Democrats to unite behind her. Vice President Joseph R. Biden Jr. and Senator Elizabeth Warren of Massachusetts also endorsed Mrs. Clinton. Senator Jeff Merkley of Oregon, the only senator to endorse Mr. Sanders, told CNN on Friday that he now supports Mrs. Clinton.  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.  Besides defeating Mr. Trump, advisers say his focus is to get his ideas, like universal health care and free public college, reflected in the party platform. Refusing to concede and release his delegates to vote for Mrs. Clinton could be a negotiating tactic for winning concessions on the platform. If his delegates tried to nominate Mr. Sanders from the floor of the convention next month, the scene could damage Mrs. Clinton at a time she is trying to project strength and party unity.  In recent days, it had been unclear whether Mr. Sanders intended to stay in the race, and even on Sunday he did not rule out the possibility that he would formally concede the nomination in the coming days.  After he met with Mr. Obama on Thursday he said he looked forward to exploring how he could work with Mrs. Clinton “to defeat Donald Trump and to create a government which represents all of us and not just the 1 percent.” Then he held a rally that night in Washington urging voters to cast ballots for him on Tuesday in the nation’s final primary.  When asked by Chuck Todd on Sunday’s “Meet the Press” on NBC whether he was an “active candidate,” he responded that he wanted to see Mr. Trump defeated.  Mr. Sanders said that he and Mrs. Clinton planned to meet on Tuesday and that he would ask her “whether she will be vigorous in standing up for working families in the middle class, moving aggressively in climate change, health care for all, making public colleges and universities tuition-free.”  “And after we have that kind of discussion and after we can determine whether or not we are going to have a strong and progressive platform,” he said, “I will be able to make other decisions.”  There have been signs that he was winding down his run. While Mrs. Clinton has been hiring campaign workers, Mr. Sanders started laying off at least half of his campaign staff members last week. He has let go of a number of advance staff members who help with campaign logistics, as well as field workers who have been canvassing for votes.  According to a person who attended the meeting at Mr. Sanders’s home Sunday, and who spoke on condition of anonymity to describe a private gathering, there was no talk from Mr. Sanders about trying to win the nomination. The group was keenly interested in how the senator’s meeting with Mrs. Clinton on Tuesday will turn out, and whether he would get assurances that she would fight for his ideas, this person said.  While he is effectively no longer a threat, Mrs. Clinton and the Democrats are counting on Mr. Sanders to eventually get behind her candidacy. He has a loyal base of more than 10 million voters and an enormous donor list that Mrs. Clinton will want to tap into. Some of his supporters say they will not vote for anyone but Mr. Sanders, so Mrs. Clinton’s success may depend on how vocally the senator supports her.
Mr. Sanders told reporters at his home in Vermont that he had no intention of leaving the race, though he did recognize the long odds he faced.