Although FDR was supported by African Americans in the North, he was supported by segregationists in the South. Yet, he was a great president. With an isolationist in the Oval Office, the non-democratic countries might have obliterated enough democracies that the United States might have been alone in the end.
https://en.wikipedia.org/wiki/Franklin_D._Roosevelt
In his Third Inaugural Address on January 20, 1941, Roosevelt declared:
"We know [democracy] cannot die—because it is built on the unhampered initiative of individual men and women joined together in a common enterprise—an enterprise undertaken and carried through by the free expression of a free majority."
https://www.presidency.ucsb.edu/documents/third-inaugural-address
Preparing America for World War II: Franklin Roosevelt, Isolationism, and America First