Blankfein asked Bloomberg during an on-stage discussion at Goldman Sachs' Sustainable Finance Innovation Forum when the next presidential election will begin. "In 15 minutes?" Blankfein joked.
The former mayor said it would start right after the mid-term elections, which would indicate which party was ahead. Bloomberg then described how the emerging candidates were being pulled to the political right and left. "A whole bunch of people have dropped out of both of these parties," he said.
"We know where that middle is, Mike," Blankfein then joked.
"I'm going to need about 63 million of you," Bloomberg said. "In New York City, if I had 90 percent approval rating when I left, it still meant 840,000 people still hated me."
Bloomberg was mayor of New York City for three terms and considered running as an independent candidate in the 2016 presidential election. He decided not to run for fear a three-way race would give Trump a better chance at winning.
Bloomberg still heads his financial data company and Bloomberg Philanthropies and is the UN Secretary-General's Special Envoy for Cities and Climate Change.
Blankfein has indirectly criticized President Donald Trump on Twitter. Blankfein's first tweet on June 1 said the Trump administration's decision to withdraw the U.S. from the Paris climate agreement was a "setback for the environment and for the U.S.'s leadership position in the world." Subsequent tweets have also trolled Trump's plans on infrastructure spending.
Separately Thursday, as the Republicans rolled out their tax proposal, Blankfein said in a Reuters report that a good U.S. tax plan will have "stimulative elements" and could get the U.S. to higher than 3 percent GDP. The initial reading on third-quarter U.S. GDP last week showed better-than-expected 3 percent growth.
— CNBC's Christina Wilkie contributed to this report.