The Bernie Sanders campaign's former press secretary Briahna Joy Gray told the Atlantic in an interview Thursday that Joe Biden has not done enough to secure her endorsement, or the votes of many progressive Democrats.

Why it matters: Sanders, who endorsed Biden five days after suspending his own campaign, has said that it would be "irresponsible" for his supporters not to back the Democratic candidate over President Trump in November.

The big picture: Gray, like other influential Sanders surrogates, stressed that progressives still have leverage to move Biden to the left until the primary season officially ends. She said her vote is "contingent on whether Joe Biden supports Medicare for All, canceling student-loan debt, canceling medical debt, having a wealth tax."

What she's saying: Gray explained the thinking behind a tweet earlier this month in which she disagreed with Sanders' endorsement of Biden, telling The Atlantic that her endorsement would be neither "politically beneficial or, frankly, ethically appropriate."

Gray called it "frustrating" that leaders of the progressive movement are supporting establishment politics without gaining concessions on policy issues. "If these are in fact existential issues, then we need to behave that way, and not stop fighting," Gray said.

"Biden is still only the presumptive nominee, and not the actual nominee. There is still room to move his positions without actually jeopardizing the candidate in a general-election contest."

"Pushing Biden to the left makes him more electable. If he’s banking on securing independent voters, then he should be aware that a majority of independents are for Medicare for All, a wealth tax, a number of other so-called progressive policies that Biden has, up until this point, strongly resisted."

Gray disagreed with the notion that refusing to support Biden at this point will only strengthen Trump's chances in November:

"No matter what progressives do, we’re going to get framed as somehow responsible for any negative outcome," Gray said.

"The Democratic Party is telling us, it seems, that they’re more interested in shaming voters than actually putting forth the best nominee."

Go deeper: How top Sanders surrogates feel about his endorsement of Joe Biden