"There's a great deal in Mike Bloomberg's record that is a real problem for very many Democratic voters," said Frank, the former Massachusetts congressman who retired in 2013 after more than three decades on Capitol Hill.
Frank, a CNBC contributor, said that Bloomberg has long opposed financial regulations and raised money for Tom DeLay, the Republican former House majority leader who was convicted on money laundering charges in 2010.
Additionally, as mayor, Bloomberg supported the police department's "stop-and-frisk" policy that disproportionately targeted minority men. Earlier this month, he apologized at a black church in New York, saying he was "wrong, and I'm sorry."
As the campaign goes on, Frank said "there's even more negative about him, more that will come out."
A Bloomberg spokesman responded to Frank's comments by pointing to the billionaire former mayor's accomplishments and policy priorities.
"Here is his record: the most successful advocate for gun laws and combating climate change, a champion for immigrants, public education and college affordability, a leader on reproductive rights, instituted some of the most innovative and forward-looking anti-poverty measures in the nation, a proven job creator in the public and private sector and more," Marc LaVorgna told CNBC. "He has record of breaking down structural barriers to deliver results on core democratic issues and he will be the strongest candidate to ensure we beat Donald Trump in the general election."
Bloomberg, founder of the Bloomberg media company, announced his presidential bid on Sunday. The former Republican is currently polling in No. 7 with 2.5% of support, according to RealClearPolitics.
However, Frank said that having a high-profile name makes sense when it comes to his rise in the polls. What's more important is the people who see him negatively, he added.
"If you were to measure negatives, he would have by far the biggest negative," Frank said in a "Squawk Box" interview.
Bloomberg has the highest unfavorability rating, 25%, of any Democratic candidate among among primary voters, according to a Morning Consult/Politico poll from Nov. 10. However, that same poll put him 6 percentage points ahead of President Donald Trump in a hypothetical 2020 matchup.
At the same time, a former advisor to Republican former presidential candidate Sen. Mitt Romney told CNBC on Friday that he sees Bloomberg changing the dynamic of the primary. Though he doesn't see Bloomberg winning the nomination.
"Inevitability, what his money and what his involvement will mean is it will return the throw way of the party back toward the center of this race," Lanhee Chen said, referring to left-wing candidates Sens. Elizabeth Warren and Bernie Sanders who are polling at No. 2 and No. 3. "I don't think it's going to be the case that he's going to pull things to the left."
The Bloomberg campaign said it will raise taxes on the wealthy, expand health insurance without eliminating private plans and take action on gun control and climate change.
Bloomberg, whose fortune is estimated at $54 billion, has already poured millions into advertising campaigns. He recently launched a $30 million-plus television ad campaign that will run through at least Dec. 3.
"The interesting dynamic with Bloomberg will be, once his advertising starts to take hold, once people start to see his liberal bonafides around gun control, around regulation, around climate change, I think his numbers will improve," Chen said.
"I think Bloomberg is going to be a factor in this race, no doubt about it," Chen added.