I'd add, for the benefit of any followers of this thread who happen not to be deeply familiar with Marx's thesis of general proletarianizing alienation laying the groundwork for revolution, that Marx predicted, accurately, that the routine scut work of managing industrial enterprises would pass from actual owners to hired managerial and technical experts. These people make up what we now call "the middle class" for the most part. The classic liberal 19th century middle classes were people who had independent means--not sufficient wealth to invest and live off of profits alone, but neither were they employees of other people. (They also would typically employ servants--just a few, one or two housekeeper-maids, maybe a nanny--but you have to get pretty high up the professional ladder nowadays to be able to afford to do that, whereas if you read many a book from the 19th or early 20th century, having some kind of hired help was just plain normal for the middle classes). All that changes when society gets thoroughly industrial-capitalist; objectively speaking our modern "middle classes" are in fact glorified proletarians, utterly dependent on salaries to survive year after year. Their resumes might get them another well paying job pretty quick and their savings might last them a few years between such gigs, but they are dependent.
In the 1930s OTL, the transition was not quite as far along as it has been in the past generation or so, which have witnessed such traditional middle class professions as medicine or lawyers being proletarianized too, drawn into hand to mouth dependency on HMOs and big law firms, the lawyers also undermined by legal software, but aside from such professionals persisting in the older model of independent real middle class status, the transition to a professionalized and dependent middle management of firms, along with cartelizing a lot of nominally independent businessmen into franchised chains where their independence was largely a paper fiction, was well along.
In real life OTL, there are mechanisms aplenty to coopt such objectively proletarian strata into loyally taking the property owners' point of view; at a certain limited rate, at high enough levels they even accumulate enough property to start objectively transitioning them to small-fry capitalists in their own right, and the prospect of such rewards is good enough to keep most toeing the line pretty effectively. I daresay these mechanisms are pretty effective even in the ATL. But there is a world of difference between a society in which essentially everyone who rises to the level of managing a plant is reliably anti-Communist, and one where even only a minority, but not a vanishingly tiny one, has at least some Red sympathies.
Meanwhile we have examples such as Patton and Eisenhower being secretly radicalized party comrades subverting the high officer ranks of the Army to show how far the "rot" of subversion has spread upward.
The ATL has the actual proletarians very heavily radicalized, so that the number of conscious socialist or outright communist revolutionaries are comparable to the numbers of proto-Patriots in the American Revolution of the 1770s--somewhere approaching a quarter the population or so are some kind of leftist radical. Now as we rise above that level, to white collar executives going places, normally those numbers would drop precipitously even in a society with such a reddish base. Still, it makes a difference that they don't drop to vanishing one part in 100,000 or so levels, but to say one percent or even more.
Part of the Marxist prediction is that management itself is proletarianized in that much of the scutwork can be delegated very far down the ladder to people paid not a lot more that shop floor workers. Given that, with absolutely no help from people with managerial titles, they can in the context of a general socialist paradigm, muddle through pretty well to keep the factory wheels turning under worker control. But in Reds! America, they can readily get the help of people who do have managerial titles, insofar as some pro tips from them are helpful.
Throw in the fact that the crisis breaks in 1933, a year where even OTL a return to pre-Crash prosperity was hardly around the corner and the desperate attempts of Herbert Hoover and others in his circles to try and jump start it earlier had failed so sweepingly that the claim it might ever return was starting to look like a sick joke. Millions of people who otherwise might be staunchly against Red revolution were down on their luck, turned out as redundant or having seen the entire corporate structure they worked for go under and lock its doors pending better times.
Thus--once, as our author has said, the Reds begin to win the morale battle, all across the nation (more or less, less so in some pockets of reaction) business switches over to worker-managed bases pretty quickly and smoothly, and at least a few high level managers would throw in with them pretty quick, and some with some enthusiasm. Then a bandwagon effect builds up whereby the majority of skilled managers and engineers who might have been leery of being leveled down with the mob they'd worked hard to elevate themselves above start to reflect on the probable prospects for the last old regime manager types to get any kind of respectable position and the stampede to rally to the Red flag among them begins in earnest. So the revolutionaries can manage pretty well without the "middle class" middle managers, but also can get their services well enough and with some desperation on their part too. At least a few will have been known comrades from before, many more will impress with their diligence, leaving the rest to figure out how to parley their skill set into winning over the worker's councils as best they can.
The majority of working people who might not have thought of themselves as revolutionaries at all will find it easy to go along with the new order, which is authentically run by their peers. As long as the workers can keep the lights on and the essential commodities flowing, and with some considerable margin for forgiveness of temporary shortfalls due to ongoing civil war, the hold of Red revolutionaries on the infrastructure machinery of society is pretty acceptable and pretty pervasive right from the start. Few of the kinds of disruptions Russia suffered in their Civil War will apply. There, the cadres of old Bolshevik workers were the ones most immediately decimated as they were the first volunteers for Red militias and the Red Army; here too that is true, but there are lots of highly skilled workers with a pretty good grasp on how to keep the wheels turning, enough that substantial numbers can be killed off but leaving still plenty to keep the infrastructure rolling along.
And it is precisely per Marx that the most developed, intensely industrialized zones have the most intensively alienated and organized workers, and they are situated right at the nerve centers and main muscle of the national industries. Reactionary regions are almost by definition peripheral and relatively deindustrialized, separated from each other, their former channels of communication seized by the rebels--who let us recall, were also the legitimate winners of a properly bourgeois rules conducted election which had its outcome nullified by reactionary violence. They hold the moral high ground as well as command of the industrial core.
One could have the luck turn on the rebels, have some centers of reaction seize key centers of communication and industry, and have the revolution turn into a destructive knock out fight. But these revolutionaries have been preparing for some time; this does not make them infallible but does mean they have fair odds of relatively quick success, and once they start succeeding it tends to snowball for them, and against their foes, who are driven into increasingly peripheral zones as the most crucial are seized by Reds who follow through going down the priority ladder. Since many secondary and tertiary targets would have been spontaneously lost to the reactionary side early on, this process of consolidating victory has a leg up and only proceeds the faster as people caught in reaction-controlled territory can foresee victory for the proletarian side, which will embolden them to commit acts of sabotage, whereas reactionaries will be demoralized and reflecting more and more on which is less bad for them, to lose and be driven into exile if not shot outright, versus trying to cut a deal with the rebels before their leverage dissolves out from under them completely.
I imagine part of the power base of the right parties in the new order will be individuals who gave in only reluctantly on the basis of concrete promises made for a less bloody takeover of specific bits of territory that the Reds are honor bound to keep faith on. It buys some tolerance they might prefer not to have to extend. But this is discussion of the longer term dynamic, right now the question is about the initial civil war.
And it is no minor point that Patton and maybe some others have contingency plans in place to hamstring the military elements they cannot win over right away. It won't take followers of Marshall and MacArthur long to realize they've been hacked and start doing ad hoc end runs around the standard machinery, but that too sows confusion regarding the actual chain of command. The initial confusion and demoralization buys the Reds time to take power directly and also to pitch appeals to military forces that might otherwise follow orders and come in to fight them; with so many soldiers recruited from such a reddish population, it is a good bet that many a massacre which would have gone "well" on paper was preempted by commanders either surrendering or switching sides actively. This in addition to sleeper cadres Patton had been cultivating!