Whereas progressives endorse and have implemented deliberate and systemic racism, and give it the name affirmative action.
If we rely on King's standard of judging men by the content of their character and not the color of their skin, then it stands to reason he would be as displeased with democrats as he would republicans.
No, he absolutely would not be as displeased with Democrats as he would Republicans. Please don't equivocate.
MLK believed that improving racial equality involved redistributing power and money in favor of the less fortunate. Republicans oppose this.
MLK preferred peaceful protest, but understood why people riot and that the solution was to fix the social issues that led them to riot. Republicans are about order for order's sake.
MLK objected to whites who argue that black people should protest in a "more convenient season." Republicans are the ones telling black people that they shouldn't kneel at football games (or on the streets, or...)
MLK railed against excessive materialism and militarism. Republicans love materialism and militarism.
Modern Republicans are curiously reluctant to condemn racist attacks, or a President who suggests there are "very nice people" among white supremacists. And let's not forget: KKK members and neo-Nazis
love Trump.
And lastly: the bill to make MLK Day a holiday was primarily opposed by Republicans; all the currently sitting politicians who voted against it are Republican. (I suspect he'd have been too modest to want a day for himself, but nonetheless)
I'm sure MLK would have issues with modern Democrats, but based on the issues and voting records, there's no way in hell he'd say they were as bad as Republicans.