The Germans came within 20 miles of Moscow. It's far from impossible that they could have invested a siege. However, there's a big difference between that and actually controlling the city and a Battle of Moscow could easily have become a bigger and earlier version of Stalingrad. That said, and while I'm not as familiar with the details of the American Civil War, defence was probably easier in the 19th century when those attacking had less firepower at their disposal.
In either case, it'd be more likely to come down to a political or social capitulation rather than a military one - and on that score, the Soviets were the more vulnerable - unless the defenders did something pretty stupid and the attackers took rapid advantage. However, there are enough examples of stupidity throughout history for it not to be ASB.
In either case, it'd be more likely to come down to a political or social capitulation rather than a military one - and on that score, the Soviets were the more vulnerable - unless the defenders did something pretty stupid and the attackers took rapid advantage. However, there are enough examples of stupidity throughout history for it not to be ASB.