Although aides to both campaigns have said an endorsement is coming, the Sanders advisory allows only that he and Clinton will discuss “their commitment to building an America that is stronger together and an economy that works for everyone, not just those at the top.”
Meanwhile, two progressive groups that had backed Sanders in the primary but have held out on supporting Clinton are formally moving into her corner.
The Communications Workers of America announced Monday morning that it would "wholeheartedly" endorse Clinton.
The union warned that if Donald Trump is elected president, union workers would be "watching our backs for years."
"Hillary Clinton is thoughtful and experienced. Donald Trump is reckless, unthinking and much more likely to cut a deal with his billionaire colleagues than look out for working families," the union said in a statement. "The choice is clear."
And another group that had backed Sanders, the Progressive Action Political Action Committee (Congressional Progressive Caucus), also endorsed Clinton on Monday.
"“Hillary Clinton shares our ideals and has worked to improve the lives of Americans her entire adult life – as an activist, a litigator, a First Lady, a Senator, and then a Secretary of State," the PAC said in a statement. "Members of PAPAC unite behind her now, and we will work to ensure Secretary Clinton is the next President of the United States."
The organization is led by two of Sanders's biggest congressional endorsers, Arizona Rep. Raul Grijalva (who has already endorsed Clinton) and Rep. Keith Ellison (D-Minn.).
Both endorsements signal that progressive groups are urging their supporters to get behind Clinton ahead of a likely endorsement by Sanders.
Although Clinton effectively clinched the nomination more than a month ago, Sanders has been slow to formally endorse her fall bid against presumptive Republican nominee Donald Trump. He has instead maneuvered to win commitments from Clinton and the Democratic Party to incorporate portions of his agenda into theirs.
Last week, Clinton announced revamped policy on college tuition and health care that did just that. And at a meeting on the Democratic Party platform, Sanders successfully pushed for liberal positions on an array of issues, including the minimum wage and climate change.
A statement put out by Sanders on Sunday called the platform the “most progressive in party history.”
This post has been updated.