The senator predicted widespread support for the Biden-Harris ticket, even as his supporters continue to lobby for progressive priorities.
"Obviously, there may be disagreement," he told ABC Chief Anchor George Stephanopoulos. "A lot of my supporters are not enthusiastic about Joe Biden. You know why? I ran against Joe Biden. But I think there's an overwhelming understanding that Donald Trump must be defeated, Biden must be elected, and that the day after he is elected, we're going to do everything we can to create a government that works for all of us."
On Monday, Sanders is scheduled to speak at the Democratic National Convention, including to the over-1,100 delegates he amassed while finishing in second place during the party's presidential primary
On "This Week," Stephanopoulos asked Sanders if, despite his own campaign's disappointment, he would still "declare victory" when he speaks at the convention, given the way in which the party has shifted to the left in the five years since launching his first presidential run.
The senator first pointed to election victories as evidence that "the progressive movement has been making enormous progress." The party's left wing has scored several congressional primary victories this year, including upsets of formerly entrenched incumbent Democrats, like Reps. William Lacy Clay in Missouri and Eliot Engel in New York, by progressives Cori Bush and Jamaal Bowman, respectively. But Sanders also pointed to growing support for a number of progressive policy proposals.
"On all of the ideas that we have been campaigning … understanding that health care is a human right, the need to raise the minimum wage to at least $15 an hour, the understanding that climate change is an existential threat and that we can create millions of jobs (by) transforming our energy system," Sanders said. "We have made enormous progress in bringing the American people in our direction, especially the younger generation."
Stephanopoulos challenged the senator, however, on a tentpole of his presidential platform that did not gain an official endorsement from the Democratic Party, nor Biden.
"Not quite 'all' the issues. Not 'Medicare for All.' And some of your supporters aren't happy about that," Stephanopoulos said, noting two prominent supporters of his presidential campaign, Reps. Ro Khanna, D-Calif., and Rashida Tlaib, D-Mich., among hundreds of others who have both spoken out on the subject and announced that they will vote against the party platform.
Sanders said his message to those who may be disappointed was that they would "continue to fight for Medicare for All," even after a Biden election, but he was pressed further about acolytes dismayed by Biden's selection of Sen. Kamala Harris, D-Calif., as his running mate, such as his former campaign press secretary who said the selection showed "contempt" for the Democratic base.
"Kamala Harris endorsed Medicare for All during the primaries," Stephanopoulos continued. "Do you believe she's an ally in this fight for Medicare for All?"
"Well, I believe that Kamala is -- as somebody who has known her for a number of years -- incredibly smart, incredibly tough. And I would not like to be Vice President (Mike) Pence in a debate with her," the senator responded, without directly answering the question. "I think she's an asset for the Biden campaign and I think she's going to do great on the campaign trail."
Sanders also commented on the ongoing battle over reforms at the U.S. Postal Service and the impact they may have upon widespread vote-by-mail efforts this fall. The senator has called upon Postmaster General Louis DeJoy to resign and pushed back against Trump's assertion that Democrats were unwilling to provide funding for the postal service to operate at its normal capacity.
"Three months ago, the Democratic House of Representatives passed a HEROES bill which would fund the post office, make sure that workers in this country continued to get (a) $600 supplement to their unemployment insurance if they're unemployed, get at least $1,200, and provide massive help to cities and states," he said. "To say that the Democrats are not working on behalf of working people or not funding the post office is another lie from Donald Trump."
He also criticized Trump and Senate Majority Leader Mitch McConnell, R-Ky., for not involving themselves in the aforementioned stalled coronavirus relief negotiations.
"Who's ever to blame, it's still a stalemate. So my question is, what do the Democrats do at this point?" Stephanopoulos challenged. "Do you have to come back, cancel the recess right now and put more pressure on this issue -- on the president and his team?"
The senator agreed with House Speaker Nancy Pelosi, D-Calif., who, House Democratic aides told ABC News is considering cutting her chamber's August recess short to address the subject. Sanders then offered additional suggestions.
"I think we've got to do everything we can to get rid of this new postmaster general who is clearly a campaign contributor for Trump, trying to undermine the Postal Service," he continued, referencing the millions of dollars DeJoy' has given to the Trump Victory Fund and other GOP candidates. "And make it clear to the American people, whether you are a progressive or moderate or a conservative, this goes beyond political ideology."
ABC News' Benjamin Siegel contributed to this report.