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We’ve all heard about how the CIA meddles in foreign countries’ politics, but let’s flip that around. How have foreign intelligence agencies influenced US politics?(r/AskHistorians)
During the Cold War, the Soviets used segregation and other racial discrimination in the United States against the US in their propaganda. It was a big problem, because 1) it was true, 2) the USSR and the US were competing for the allegiance of many non-white nations that were just at that moment decolonizing and coming out from under the rule of whites (think African diplomats from decolonizing nations who are making their way to DC being forced to the backs of buses and not being served in restaurants while a harried and immensely embarrassed Secretary of State personally intervenes to get said diplomats to DC in time and with a reasonable impression of the US), 3) it made the US, with all its rhetoric on freedom, stink of hypocrisy.
This led to a big turnaround in executive policy and legitimized the beginnings of the civil rights movement. The Justice Department started submitting briefs in SCOTUS cases in favor of desegregation, with the Brown v. Board brief containing several pages discussing the national security implications of continued segregation in the US. It’s a bit of leap to say that it was directly because of such briefs that SCOTUS eventually ruled Brown v. Board as they did, but I think it’s plausible to say it was nonzero. The quite muscular show of federal support for Brown v. Board after SCOTUS decided it, such as at Little Rock, is further indication of how big of an issue it had become for the executive, from which you can induce that Soviet propaganda/national security with regard to segregation/discrimination had become a big ass deal.
More broadly and abstractly, this Soviet propaganda pushed desegregation and the civil rights movement more generally closer to the mainstream, protecting the emerging movement from the kind of accusations of communism that labor forces fell victim to during the First Red Scare.
Source: Anything by Mary Dudziak. I can edit in specific citations if you want.
Note: Oh lord. I just reread the question and noticed you wanted /covert/ influence on US politics. Well, I think this is closely-enough related and may be of enough interesting to you for me to post it anyway. Is that a no-no in /r/AskHistorians?