For an interesting appraisal of the true significance of the 1919 split, see Jack Ross, *The Socialist party of America: A Complete History* (an admittedly biased and in some ways eccentric work but still the only comprehensive history of the SP):
"If anything approaches a consensus view of what caused the decline and collapse of the Socialist Party of America, it is to identify it in th split that caused the Communist Party in 1919. It is certainly true that, as James Weinstein declares in the final sentence of his brilliant study, 'The legacy of 1919 was the alienation of American Socialism.' But because it ultimately proved to be the *legacy* of 1919 does not mean that the Communist split is what made the decline and collpase of the Socialist Party a *fait accompli.* Indeed, Weinstein illustrates this more clearly than anyone, giving the attention of the Labor Party movement as the Socialist Party unfolded and illustratring the causal relationship between these events.
"An empirical measurement that may foster confusion between the ultimate legacy and immediate consequences of the 1919 split is the rise and fall of dues-paying membership. As early as 1913, nearly 30 percent of the total SP membership was affiliated through the foreign language federations. By 1917, when the average membership was 80,379, the number of language federation members had only modestly increased by about 5,000, but now constituted over 44 percent of the total membership. At the most inflated post-1912 peak of membership-—104,822 in the first quarter of 1919-—the language federations constituted 54 percent of the membership at 56,740. Excluding the language federation numbers, which rapidly evaporated after the founding of the rival Communist parties, the SP membership average of 34,926 in the fourth quarter of 1919 represents a serious, but, in proper perspective, relaticely modest loss of 13,156, or 27 percent, from the beginning of 1919, when there was only a slight increase in non-federation membership of roughly 3,000 from 1917.
"Tellingly, even among new left historians the one-dimensional attribution of the Socialist Party's decline has distracted from asessing the real impact of government repression during the war, which James Weinstein is second to none in forthrightly describing as 'a reign of terror far worse than any conducted in Europe, either among the Allied Powers or within the German Empire.' The impact of the war, the Communist split, and decisions made in the months after the split must all be given their due in diagnosing the collapse of the Socialist Party. But one unmistakable fact balances the scales to decisively assign blame to wartime repression: the two places where a formidable party organization survived through the 1920s, New York and Milwaukee, were the two large cities where banning from the mails was not a death sentence for a viable party press.
"It also bears emphasis that the departure of the future Communist Party did not notably rupture either the historic base or leadership of American Socailism--potential candidates for leadership such as Charles Ruthenberg and Alfred Wagenknecht were the exception and not the rule. Indeed, the Socialist Party lost a far larger and longer established portion of its talent to the pro-war defection. Finally, although there was certainly some basic continuity from the historic left wing, particularly International Socialist Review, to American Communism, the program of the Third International nevertheless represented a fundamental break with the historic left wing. In the words of James Weinstein,
'Most of the Americans who joined the left wing had no concept of what later came to be called the Leninist party. Quite the contrary, they had traditionally opposed Hillquit and Berger as bureaucrats, and had advocated greater decentralization and autonomy. . . . The native left-wingers who went Communist did so out of romantic identification with the Russian Revolution, and ecause of the panicked, bureaucratic action of the Old Guard in expelling the foreign language federations in the spring of 1919. But few of them could remain for long in a party that boasted, as Alexander Bittelman did in 1924, of its ability to change its line in 24 hours at the behest of the International...'"
https://books.google.com/books?id=fud1BwAAQBAJ&pg=PR135
https://books.google.com/books?id=fud1BwAAQBAJ&pg=RA1-PR1