That's way too much of a generalization. Maybe you want to rethink it. I'd have assumed that an actual Marxist approach to politics would make campaigns a lot more bitter, passionate and bloody. If "cultural Marxism" actually makes politicians behave better towards each other, people are going to take another look at it.
Political passions go through cycles. Hamilton and Jefferson savaged each other. A few years later, the Federalists had almost died out and we had the "Era of Good Feeling." JQ Adams and Andrew Jackson fought bitterly, then passions died down a bit until slavery heated them up again. As bad as that 1884 Cleveland-Blaine campaign was, it wasn't as vicious as what came earlier in the Civil War or later during the fights between Bryan and McKinley, when more was at stake.
For some people, the postwar Eisenhower-Kennedy consensus years are still the norm and what's going on now is truly abominable by comparison. You're right that past political fights could be bitter, and that there have been much more divisive campaigns in the past. All wasn't sweetness and light. But polarization and viciousness in politics rises and falls over the years.
Cultural Marxism = Political Correctness
The antithesis of free speech.
Are you under the impression that the purpose of the First Amendment was to protect only speech that no one finds offensive?