The number of people, at any time and in any nation, over the last two centuries that have partivcipated in (left) political activity has been a very small proportion of the poulation. Prbably never, even in revolutionary situations or at the height of any mass movments it has never beenmuch beyond 5% and at the very most 20%. So that can't be a factor in considering the potential for growth of a left. At present it probalby is not more than one or two pecent, and no one issue or ongoing event unites them.
One has to separate descriptions of a problem from excplanations or solutions to the problem.
If I understand Julio's conception it calls for reaching a huge number of the population to acheive anything. It would have to nominate and elect a majority in both the House and the Senate. No left movement has ever had that kind of electoral support. The left has achieved things by "persuading" non-leftist (and even anti-leftists) politicians to make changes they (a) personally did not want and (b) which they did not need to make in order to be reelected.
Carrol