I think you answered your own question.
(Emphasis mine.)
Republicans realized long ago that their grip on power in the US was on the wane, and they don’t like it. They had two choices: Broaden their appeal so they could win in the customary manner, or cheat. They chose cheat.
Once they had crossed that Rubicon, they could easily adhere to the belief that the ends always justifies the means, because Republicans in power is Righteous and Good and the American Way.
For Russia, anything that weakens America’s standing in the world is a good thing. Their interest is so transparent, and I don’t understand why more Americans don’t see it. But as I’ve said many times, that war on education by Republicans over the past 50 years is paying big dividends now.