Some, such as party fundraiser Lou Frillman, had already signed up to help Clinton's campaign.
"We believe that the vice president is the best candidate," Frillman said in an interview. "We have a lot of confidence in him....If he runs, we’re ready to support him."
A spokesman for Biden declined to comment.
Among those who signed the letter were Los Angeles software executive Yolanda "Cookie" Parker, Chicago trial attorney John Cooney, Connecticut hedge fund manager Jim Torrey and Democratic National Committee finance committee member John Maa.
In the letter, which was first reported by Reuters, supporters call the Obama-Biden administration "a spectacular success."
"To finish the job, America needs a leader who is respected both home and abroad, and who understands the real challenges facing American families," it reads. "In our opinion, the next president must be Joe Biden. If he announces he’s running, we’re all in. It’s a campaign we know he will win."
The letter does not mention Clinton or Sen. Bernie Sanders (I-Vt.), who has garnered swelling support for his insurgent bid.
Calling the vice president "an authentic leader," the Biden backers argue that "his expertise in foreign affairs is unmatched, and his knowledge of domestic policy is deep and broad."
"No one is better prepared for the Presidency than Joe Biden," the letter reads.
Here is the full text: