"Let me make it very clear if I haven't 10,000 times previously," Sanders told reporters at a press conference in Washington on Tuesday. "I think Donald Trump is totally unfit to be president of the United States."
The Democratic rivals are scheduled to have their first face-to-face meeting in months at a Washington hotel Tuesday night, just after the final ballots of the long Democratic primary are cast in the District of Columbia. Sanders said he looked forward to the meeting "very, very much."
The question is whether Clinton does, given Sanders' insistence that he will not fully step aside before the Democratic convention next month in Philadelphia.
But Sanders made clear during a closed-door meeting with Democratic senators on Capitol Hill, and later at an outdoor press conference, that he intended to shift his battle from Clinton to reforming the Democratic National Committee
"We are going to fight as hard as we can to create a Democratic Party, which represents the working families and the low-income people in this country," Sanders said.
A person close to Sanders said the meeting should be "amicable and hopefully constructive."
Sanders "respects" Clinton, aides told CNN, but plans to "put his cards on the table" and push for progressive policy positions to be included in the fall campaign and in the party's platform.
Sanders outlined to reporters which concessions he is seeking, including a change in the leadership of the DNC, more open primaries that allow independents to cast ballots in Democratic contests and the removal of superdelegates from the Democratic rules.
"The time is long overdue for a fundamental transformation of the Democratic Party," Sanders told reporters, saying his delegates were intent on seeing change. "Let me tell you what they want: They want to see the Democratic Party transform."
Sanders believes his collecting 1,850 delegates after winning 23 states is significant leverage for his views. Clinton intends to listen "respectfully" to Sanders and his call to influence the platform, an aide said.
Clinton enters the meeting hoping Sanders will help her, particularly among white working class voters and young voters. Her campaign is less concerned about platform right now than neutralizing any "unnecessary opposition" that could distract from defeating Trump.
A person close to Sanders' campaign told CNN Tuesday afternoon Sanders will likely ask for a roll call vote at the Democratic convention. Clinton herself received a roll call vote in 2008, but personally asked for it to be suspended and she formally placed Obama's name in nomination as a sign of unity.
Asked Tuesday if he would similarly nominate Clinton in July, Sanders declined to answer.
Sanders is returning to Washington as a new leader of the Democratic Party. He arrived to applause from fellow senators Tuesday at their weekly Democratic policy lunch in the Capitol. He was later welcomed back to the Senate with a standing ovation, Delaware Democratic Sen. Chris Coons told CNN.
"Think it was a very constructive chance for the caucus to welcome Sen. Sanders back. He got a standing ovation. Spoke movingly and passionately about the millions of people he's connected with," Coons said.