The senator's speech encapsulates a growing hostility towards immigrants in 1920s America. Society was changing rapidly, and many of those bewildered by the various changes sought to scapegoat immigrants for things happening in society that they didn't like.
The United States had always been a nation of immigrants, but the whole complexion of immigration underwent significant change in the late nineteenth and early twentieth centuries. A large percentage of this fresh influx came from Eastern and Southern Europe; the vast majority of immigrants from these areas didn't speak English. Also, a growing number were Jewish, many of them fleeing persecution.
Hostility towards these newcomers is reflected in Smith's speech when he claims, among other things, that they would become a burden on the public purse. This was, and is, a common trope in anti-immigrant rhetoric, as is Smith's suggestion that the recent wave of immigrants cannot be fully assimilated into society on account of their different language, culture, and political traditions.
To an avowed racist and segregationist like Senator Smith, such people represented a threat to what he considered to be the traditional American way of life. They spoke strange languages; they had unusual customs; they came from places which didn't have the same tradition of government as the United States. As such, they constituted a danger to the republic. They weren't what Smith and many others would describe as "100% American." Indeed, it's notable that Senator Smith makes no attempt to hide his true feelings on the subject, openly referring to breeding what he calls "a pure, unadulterated American citizenship."