He also insisted -- despite the fact that Sanders trails Clinton in both pledged delegates and superdelegates and faces a nearly impossible path to the nomination
-- that there will be a contested Democratic National Convention.
"The Democratic Party must decide if they want the candidate with the momentum who is best positioned to beat (Donald) Trump or if they are willing to roll the dice and court disaster simply to protect the status quo for the political and financial establishment of this country," Weaver wrote.
He cited a "scary" Quinnipiac University poll
of swing states Florida, Ohio and Pennsylvania showing tight races between Clinton and presumptive GOP nominee Donald Trump -- races that Sanders performed slightly better in, though within the poll's margin of error.
"Because we must do everything we can to defeat Trump in November, our mission is to win as many pledged delegates as we can between now and June 14," Weaver wrote. "Then we're going to have a contested convention."
The Clinton campaign had no immediate comment on Weaver's statements.
Weaver addressed blowback to the fundraising email in an interview on CNN Thursday. He said the "disaster" he was referencing in the pitch was a Trump victory, and was not meant as a criticism of Hillary Clinton.
"The disaster is not Hillary Clinton. The disaster is the election of Donald Trump," Weaver said. "Look, electing Trump to the presidency of the United States would be an unmitigated disaster. "
Weaver also defended the campaign's decision to continue its long-shot White House bid and argued that Sanders' presence in the race would not harm Clinton, but would keep progressive issues at the fore of the national political debate.
"Believe me, as soon as this nominating process is over, regardless of who is the nominee, I can guarantee the media coverage will be about name-calling and finger pointing, and we're not going to have any more discussion about the issues. As long as there is a democratic process going on, issues of importance to Democrats are being discussed," Weaver said.
The Sanders campaign has said it will continue to fight on in the remaining primary states as well as trying to flip Clinton superdelegates -- who are unbound until they vote during the convention -- while also planning efforts to shape the party platform.
"I will continue to run an issue-oriented campaign. Will I be taking on Donald Trump? Absolutely. Will I be discussing the differences of opinion Secretary Clinton and I have? Yes, I will," Sanders told CNN's Wolf Blitzer on Friday.
But just as Sanders' supporters clamor for their candidate to continue his long-shot bid, party leaders -- including members of the Obama administration -- have signaled they think that extending the primary contest does more harm than good.
"I think everybody knows what that math is," Obama told reporters last week, referring to the delegate count that shows scant opportunity for Sanders to overcome Clinton. "At some point there's going to be a conversation between Secretary Clinton and Bernie Sanders about how we move towards the convention."
And Vice President Joe Biden went even further, saying in an ABC interview this week that he felt "confident that Hillary will be the nominee and I feel confident she'll be the next president."