From: Lochan nan balgair-dudh
Would there have been an effect upon Papa Joe's regime? Don't think so. Papa Joe and Company absorbed terrific losses and still met out some serious hurt on the Axis up to Stalingrad and then whipped the Axis in every way. (So I think.)
I'd disagree with this. Leningrad was 'Lenin's city', 'birthplace of the revolution' etc, it held a place in the glue that kept the Stalinist regime together second only to Moscow. By November 1941, the Communist Party was no longer in control - Stalin was governing via the GKO-Stavka, at a local level the army was running things - now this was in part as a lot of party cadres (too many as some were ordered back to the factories) had joined up - and the pre-war civilian governance had collapsed.
Some Soviet generals were talking openly of basing the regime (note they were still talking about keeping the Stalinist state) on the army not the party. If you dropped the loss of Leningrad into that, it would have been a profound shock to the regime's legitimacy in late 1941.
Note I'm not suggesting that it would have a material impact on the Russian capacity to wage war, nor that they lost a lot of resources keeping the city (just) alive, but that in psychological and political terms, given the state of the Soviet State in late 41, then yes, it could well have been the final thing that toppled the pre-war Soviet state.