^^^ CB; My argument for this relies on seeing the famous slogan "Workers of all countries, unite !" as fundamental Marxism. That seems fairly non-controversial, given the prominence Marx and Engels gave the slogan.
Next, we note that racism is essentially an ideology developed in capitalism as the very opposite of the workers of all countries unite , slogan. It is said that Lenin expanded the slogan to "Workers and all oppressed peoples of the world , unite!" Perhaps the anti-racist essence of this is clearer.
Divison of the working class, within in the U.S. and really throughout the world, is especially based on racial divisions, "lighter vs darker". "Workers unite " makes the ending of racism the number one job of Marxists.
In the U.S. , where oppression of Black workers has been especially significant, we had the slogan "Black and White , unite and fight !". The analysis was that liberation of Black workers is central to the unity and liberation of all workers. Again, Marx said in _Capital_ I " Labor in white skin will not be free, while labor in the Black skin is branded, " making the same point in a different way.