Socialist-communist ideology had always promoted the idea that non-socialist parties would "disappear" in a way. When the Communists came to power in Russia, they simply forbade all opposition parties (one after the other, forming alliances with other parties for to forbid just one (the most rightwing) party at a time). This was one of the tricks the Fascists learnt from the Bolshevists.
So, after WW2, things were not as easy. Many people seem to believe that Eastern Europe and East Germany went from Soviet occupation directly into a pro Soviet dictatorship. This is completely wrong. The Soviets at first had to permit all sorts of parties and there was an afterwar time of strong debates and real conflicts until the Communists got the upper hand. Those times 1945-1949 were rather similar to the Red Decade (1930-40) in the West or to the Sixties amd Seventies, only that there and then the Left got the upper hand at last.
How did they do it? They seduced, blamed, shamed and bullied the opposition parties into a coalition with the socialist-communist party, a coalition which was called "United Front against Fascism" or similarly. Seducing, blaming and shaming was rather a task of the media. Bullying was a task of organized groups. If a public figure expressed some non-progressive ideas, the socialist-communist groups organized strong protests and demanded his cancelling - until the United Front was fixed, thereafter the riots and pressure groups disappeared.
But this could only be effective because an important part of the oppositional party establishments kowtowed. The party bases were never asked and
later on accepted the state of affairs teeth-gnashingly. Non-communist parties survived, but they had become an echo, not a choice.
To get a faint idea, Americans could look at the situation around 1940. We have here the same combination of seducing-blaming-shaming-bullying w.r.t America First and the Republican isolationists. And the kowtow of the Republican establishment with the candidature of Wendell Wilkie, the prototype of what is now called a RINO (Republican in name only).
But we must also consider the differences. The Leftists in America then used the same basic strategies as later on the Leftists in Eastern Europe, but their temporary success in America depended on incidents the Left could not control. Crushing America First would not have worked without Pearl Harbour, establishing Wilkie not without the impasse of the other Republican candidates. And the party base successively returned to power and within a generation the parole "a choice, not an echo" was generally accepted.
The moral of this story is that a democracy cannot survive if it does not have a real opposition party which leaves people a real choice. So I am extremely skeptical about "centrism" ( "we all should accord"). It is part of a seduction which leads us out of democracy.