In both cases, they co-opted popular left-leaning movements to engage conservative voting bases. They combined these popular movements with nationalist conservative ideologies to garner wide appeal amongst conservatives.
After winning, they then used their momentum to tear down institutional checks and balances to make sure they would hold power indefinitely, even if their support started to wane. Conservative support, in both cases, remained strong for many years, resulting in the deaths of millions of innocent non-conservatives.