In 1944 the Minnesota Democratic Party united with the Farmer-Labor Party to form the Democratic-Farmer-Labor Party of Minnesota, one of the most progressive state branches of the US Democratic Party. In 1948 its mayor of Minneapolis, Hubert H. Humphrey introduced at the national convention the resolution supporting civil rights for African-Americans whose acceptance led to a walkout by Strom Thurmond and other Dixiecrats, with Thurmond running for president against Truman. Humphrey would later become a famously progressive US senator and eventually LBJ's vice president, which dragged him down due to the Vietnam War.
He was succeeded by equally progressive Arthur Naftalin as Minneapolis mayor, a political science professor at the University of Minnesota, who served until 1969. However, for reasons that remain somewhat unclear, the attempted progressive policies of these majors did not result in excellent conditions for the city's then quite small African American population, who lived in highly segregated neighborhoods. Whatever progress did happen was substantially damaged by Naftalin's successor as mayor, Charles Stenvig, the city's police chief, who ran on a platform that demanded to "take the handcuffs off the police" and promised to crack down on "racial militants." He was reelected in 1971, and many see him being a major influence in the police department of Minneapolis becoming an exceptionally racist and vicious one.
All this is recounted in a 2008 paper that appeared in the journal American Studies by Jeffrey T. Manuel and Andrew Urban, "'You Can't Legislate the Heart': Minneapolis Mayor Charles Stenvig and the Politics of Law and Order." vol. 43, issue 3/4, pp. 195-219.
Furthermore, with African-Americans moving more into the city in more recent years, the gap between educational outcome as well income and employment outcomes between the races has increased to be among the highest in the nation, despite the liberal past and reputation of the city. These facts contribute to the bad racial situation in the city, which combined with the racist police department have led to this awful current situation there.
A source on the educational gap is mprnews.or/story/2019/10/14/mn-among-worst-achievmentgap-states , and a source on the income and employment gas is politic.com/magazine/story/2016/07/minnesota-race-inequality-philando-castile-214053 , this latter also dealing with bad racial police behavior in Minneapolis.
I thank Tyler Cowen at Marginal Revolution for these sources, and this general account, which I did not know of. This is indeed a sad tale, given the proud and generally admirable history of the Minnesota Democratic-Farmer-Labor Party.