RRWI: Mensheviks win over

This would have been difficult. Menshevik is defined as "minority" in English and they largely were. Their survival would have depended on the survival of the provisional government. Mensheviks were also split with factions taking opposing sides in the civil war. It was the Socialist Revolutionaries who held the mass support. However both the SRs and Mensheviks initially controlled the Petrograd Soviet and had they managed to retain control history may well have been different. It would probably have depended on an earlier peace settlement and Lenin not being sent into Russia on a train. Bolshevik leaders already in Russia were inclined initially to compromise even Stalin

Had the provisional government survived. I imagine events would have been a bit like those in Weimar Germany with attempted coups and left wing uprisings. Whether a Hitler figure would have merged is debatable
 
I believe they did dominate the Soviets along with the Left-SRs, the problem was their support of the war compromised them, while Lenin and his tiny band of Bolsheviks grew in favour by calling for peace at any cost. Get rid of Lenin (say the Germans don't let him through to Russia), and the Bolsheviks will be a lot less coordinated and effective, possibly butterflying the October Revolution away.

However although the Mensheviks had more popularity, they're still an urban Marxist party, they're never going to dominate the electoral polls unless in alliance with the pro-peasant SRs is solidified, and even then they'll be the leftwing fringe of the alliance. These might be pretty crude parallels but if the Russian Republic can remain, I wouldn't be surprised to see the SRs dominating as a broadly social-democratic force, ala the Indian National Congress, while the Mensheviks would form the radical (though not revolutionary) fringe much like the Indepedent Labour Party in Britain and the Independent Socialists in Germany.
 
Had the provisional government survived. I imagine events would have been a bit like those in Weimar Germany with attempted coups and left wing uprisings. Whether a Hitler figure would have merged is debatable

I can see where you're coming from with this but I really doubt the Duma would anywhere near as chaotic as the Reichstag, at least not in the first years post-war. The Socialist-Revolutionaries dominate any election and with their very simple goal of land redistribution, they're going to hold a large, loyal peasant vote for a while IMO. Also the Right is connected very strongly to the failed Empire and there's no 'stab in the back' as regardless of how peace is made, its obvious Russia wasn't going to win anytime soon. This all makes for a far less partisan atmosphere.
 
Top