"...Root was a man of his time and shared a number of his contemporaries' prejudices against Jews, Asians, and Southern Europeans. That being said, he had never been a member of the Immigration Restriction League nor ever lobbied with any particular vigor for more stringent curbs on immigration beyond the standard tests for mental acuity and sanitation, and he was personally alarmed by the explosion in popularity of the ADL over the course of the prior six months, predicting (correctly) that "within five years, the American Defense League will operate either as a transpartisan facsimile of Canada's Orange Lodge, or as a political movement untethered from electoral politics entirely."
The Immigration Act of 1918 was not some high-minded effort to head off the popularity of odious nativist groups like the ADL, however, but rather a response to a number of accelerating pressures from across the political spectrum. A backlash against the Southern and Eastern European arrivals had been brewing amongst the Protestant-majority middle class since the beginning of the decade - some historians have explained it as part of the reason for the swings towards the Liberals in 1910 and 1912 even though immigration was barely considered an issue of policy in those elections - and only the war had delayed it becoming ripe. Bottled up for three years as Ellis Island swelled with bodies to feed America's factories, farms and armies, it had detonated in the wake of the Minneapolis General Strike and the postwar economic depression, with immigrants being scapegoated for the triple scourges of high unemployment, high inflation, and high crime. Not only that, but Irish Democrats - long one of the most reliably pro-immigration constituencies - had begun to sour on the unfettered movement of people to American shores, in part due to the high social tensions but also because Irish Americans (and a good number of actual Irishmen, too) had served and died disproportionately in the American Army and were the most outraged at their difficulty at finding work upon coming home from the front. Further, the expiration of the Ingalls Act in 1913 had led to as many as a quarter of a million Chinese arriving on the West Coast during the war years and close to fifty thousand in 1917, and the refugee crisis across the Ohio showed no sign of abetting a full year after Mount Vernon.
This all coalesced into a rapidly emerging bipartisan consensus that something had to be done; moderate Liberals and Eastern Democrats persuaded themselves that they needed to defend American employment to tighten an over-abundant pool of labor, while culturally reactionary Liberals and Sinophobic Western Democrats saw in the contours of the debate around the Immigration Act of 1918 in the spring of that year the chance to enact draconian restrictions they had always advocated for. It did not take long for the Act to come together, and it was passed on May 20, 1918, after only sixteen hours of debate in the House 390-32, with all Socialists and scattered gadflies in the other two parties voting against, and it would pass the Senate two days later to be signed by Root shortly before Congress' summer recess. For the first time, the United States would have a strict immigration regime based not only on a head tax, or lack of mental or physical ailment, or rejection of criminals - but structured by country of origin..."
- The Root of the Problem: The Tumultuous Term of America's 29th President
"..the Immigration Act of 1918 fell far short of what stalwarts of the Immigration Restriction League had pushed for since the late 1890s, in part thanks to the bill being crafted largely by the conservative but pragmatic Ohio Liberal Nicholas Longworth and Ilinois' Adolph Sabath, an Austrian-born Jewish Democrat. [1] Men like Pennsylvania's Tom Butler, the Liberal House Majority Leader, or California's viciously Sinophobe Senator James Phelan were not part of its writing; indeed, Butler denounced the act as insufficient in its scope even as he whipped in its favor, and promised that a returned Liberal majority would go further at their next bite at the apple.
The Act's structure was in many ways Sabath's idea; he was, as his background would suggest, generally very favorable towards immigration to the United States but the postwar chaos had led him to acknowledge that there were perhaps upper limits on how many immigrants the country could absorb annually, both in terms of unemployment and in how easy it was for immigrants to assimilate. For such a longtime advocate against restrictionism to help write the bill to join with Longworth to do so gave the imprimatur for many other skeptical Congressmen in both parties to listen. Sabath's bill rejected any kind of literacy test, did not include provisions the deportation of "radicals, subversives and undesirables," and also did not raise the head tax; rather, it set an annual allotment of persons from every country based on the number of persons of that "national origin" who had been present in the United States in 1910. This formula massively advantaged immigrants from places such as Germany, Ireland, Scandinavia or (Sabath's home country) Austria-Hungary, while disadvantaging Southern and Eastern Europe; that was, perhaps, in part by design. This was heretofore known as the "National Origins Formula," and it would remain in place for decades as the guiding structure of US immigration policy, even as the formula was adjusted gradually over time. [2]
The 1918 Act went much further than that, however, and here Phelan's fingerprints can be found on Sabath's otherwise pragmatic compromise bill. Democrats from the Pacific to the Mississippi River had for decades been clamoring for a mass prohibition on Chinese immigration to the United States, and by the mid-1910s were vehemently opposed to Japanese immigration as well. At key junctures, such hopes had fizzled out - James Blaine had not wanted to risk his treaties with Qing China in the early 1880s, James Ingalls had watered down a blanket ban after the San Francisco pogrom, etc - but the waves of Chinese arriving in California since 1913 had triggered a violent response. The problem, of course, was that China had not long earlier gone through a revolution and become the world's largest and most populous republican democracy, extremely corrupt and flawed as it may have been, and Eastern interests were undergoing the early beginnings of an elite Sinophilia that their Western counterparts most certainly did not share. New York bankers and Philadelphia politicians saw in China "the emerging dragon of world republicanism flying alongside the Columbian Eagle's outstretched wings, counterparts on either side the Pacific," as Theodore Roosevelt put it in his New York Journal; West Coast politicians and newspapermen described the Chinese in San Francisco, Seattle or Portland as little better than rats. Western Democrats began to coalesce around a position of rejecting the Act entirely if it did not exclude Asians from American shores, and a bipartisan group of Midwestern Representatives from Missouri, Indiana, Ohio, Illinois, West Virginia and western Pennsylvania emerged shortly thereafter to reject the Act as well if it did not do something to counter the streams of refugees trying to escape Kentucky or through the hollers of the Ozarks and Appalachians..."
- The Yellow Peril
[1] An interesting fellow; at one point Dean of the House, an ardent anti-Prohibitionist who was violently opposed to the Klan, and generally skeptical of strict immigration restrictions. Dunno how realistic him drafting such a bill is, in other words, but this is my in-universe explanation for how the Immigration Act of 1918 winds up being comparatively tame (unless you're Chinese or from Dixie, as we'll see in Part II)
[2] This is how the 1917/18 Immigration bills worked; the more draconian 1924 Act, which was passed in the shadow of the German Revolution and Russian Civil War, as well as by a larger OTL Republican majority combined with Southern Democrats, adjusted that formula to be the number of people from those countries in 1890. I don't think it's possible to not have some kind of immigration backlash after the GAW, but here the 1918 formula is as far as it goes.