"I think Sen. Clinton has been sensitive, supportive of the progressive agenda, she's realistic about what she can get done, she's able to beat the Republican candidates," Simmons told CNN's Carol Costello in an interview Friday, adding that Sanders could lose.
Simmons first made the announcement over Twitter on Friday.
Simmons revealed the nature of the exchange he had with Sanders that helped him decide to throw his support behind "longtime friend" Clinton.
"Bernie Sanders is over-promising. He's insensitive to the plight of black people, and what good is it if you don't take lobbyist money and don't acknowledge the No. 1 threat to America and to the world is the factory farming lobby because it is poisoning the planet quickly," Simmons said.
When he asked Sanders about the lobby, Simmons said Sanders "brushed it off, even though he knows that your earthquakes and all these storms are a result of factory farming."
CNN has reached out to the Sanders campaign for comment. On Friday during "The Lead", Jane Sanders said, "That was an idea the he brought forward and Bernie said 'No. I'm really for the farmers.' We need to make some changes for the environment, but not the ones that he was suggesting."
Simmons, who traveled to Flint, Michigan, to raise awareness about the water crisis and donated 150,000 water bottles to families, slammed Republicans for not visiting Flint and said that Clinton's aides were there at the time of his visit.
When pressed by Costello about his comments that Sanders is "insensitive" to the needs of African-Americans, Simmons said, "He's insensitive in a number if ways, and I would get into it if we have time."
The charge that Sanders can't deliver what he promises is one that Clinton, herself, has levied against the senator.
"I don't want to over-promise," Clinton told supporters in Iowa
in January. "I don't want to come out with theories and concepts that may or may not be possible. We don't need any more of that."
Although Simmons, a critic of the Iraq War
then-Sen. Barack Obama over Clinton in the 2008 primary, he has long expressed his admiration for her.
"If you could take Barack Obama's image, add Hillary Rodham Clinton's money and John Edwards' voice, that would be my candidate," Simmons said in January 2007, a couple of months before endorsing Obama.
And in 2008, as Obama was considering his pick for secretary of state, Simmons wrote
an op-ed stating, "Clinton will be the most effective secretary of state and the best choice for the Obama team."