>>2828022They were a very popular party. In what world is a party winning 25% of the popular vote not popular? Millions of people voted for them so it's only natural that hundreds of thousands would join the party. Returning to the OP, this can also be seen in Duma elections under the Tsar where the estate-based unrepresentative electoral system set aside only a small fraction of seats to be elected by workers (with a restrictive franchise at that), the Bolsheviks won all the seats in the workers curia even then under conditions of repression and illegality. So while the party membership number may create an illusion (often repeated by modern leninists of all stripes) of the bolsheviks being this elite tiny vanguard that explodes in popularity in the revolutionary situation based on their correct theory, the truth is the bolsheviks has been the deeply entrenched party of the russian proletariat for two decades at that point, with their papers having substantial circulation and winning deputies even under the insanely unrepresentative duma electoral system. If anything the notable thing in 1917 is the growth of Bolshevik support among peasants based on their opposition to the war and support for land seizures which helps peel of the bulk of the SRs to the side of the soviets.
As for the constituent assembly elections, they obviously didn't have the support of a majority of the entire population but they absolutely did have the support of a vast majority of proletarians actually who overwhelmingly voted for the Bolsheviks, hence their very large majorities in urban constituencies, also large majorities in army and naval garrisons reflecting their overwhelming support among soldiers and sailors. The demographics of russia being as they were even getting every single proletarian behind you wouldn't be close to enough to reach 50% of the vote, the country was majority peasant and the peasants though many voted for the Bolsheviks too, overwhelmingly backed the SRs and the fate of the constituent assembly had far more to do with the Socialist Revolutionary party than the bolsheviks, since the party split during the elections with a majority of the party's base and activists being expelled for their support of soviet power and the revolution, while the rump party leadership, which in reality had now very little support reduced to the progressive intelligentsia and wealthier farmers, had written up the electoral lists and the deputies elected for the SRs didn't represent the factional split in the party. Had the Left SRs not boycotted the constituent assembly with the Bolsheviks as illegitimate and supported the Congress of Soviets instead things would certainly have developed differently. Ultimately the constituent assembly wasn't representative and ordinary people didn't care for it, the Right SRs and Kadets entirely overestimated its legitimacy in the eyes of the masses by harping relentlessly on its authority.