That sounds to me like someone who knows what American values are, and that marxist values aren't them and so he knows what American values are not. So, again, I'm not labeling you "sanctimonious"; at worst "certain he knows what are American values." I'm not sure I've ever called someone sanctimonious. I've called someone unctuous once.
Well rather than leave marxism in its current blobular form I will reason out why some nameable, main features are athwart American values and the constitution. You know, to avoid any pretensions of baseless sanctimony. Real life could strike me at any moment so if you wanted to put a point to me from all this or have the last word for the time-being, now would be a good time.
Classless and stateless society
The stateless part is fairly obvious, because the constitution establishes a government. This primary objective of marxism is unconstitutional.
Neither forming nor eliminating classes appears to me to be an American value. Since Americans are competitive, economic classes have to be expected based on the outcomes of competition. The Constitution explicitly prohibits the feudal hierarchy sanctioned by the Magna Carta (the document Tim thinks we just rephrased), but it does not intervene against competitive private activity to preserve equality of outcomes in the private sector. Accepting inequality in that form would fall under our notions of advancing freedom and individualism, I think, and it is incompatible with marxism.
Socialized ownership of the means of production
The US forces participation within specific confines we call the enumerated powers of government. FDR leveraged a crisis (may they never go to waste) to shift the battle line in Karl's favor, but there is a battle line. Capitalism invariably pulls in one direction, and marxism invariably pulls in the other.
International values
Modern strategies for spreading communism laid out by Gramsci call for using available media to supplant national values with international workers' values. This is of course incompatible with a national value or identity of any kind. There is a specific two-word term for this strategy, but modern marxists have made it so that the use of the term is prohibited in civil discourse. It's like uttering the name of Voldemort. They managed this by successfully executing one of the very tactics it prescribes: by associating it with racism.
Hence the partisan flavor of marxism in the United States: one party is rallying people by identity group, and is nurturing antagonism towards the "American identity group" by associating it with white power and racism. I am not sure which modern thinker first called for mass migration as another prong of this strategy. I only know it wasn't Pat Buchanan.