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."

The senator was expressing his opinions with regard to immigration restriction. His argument supported the idea that it was the right time for the United States to stop admitting immigrants. He cited the issue of limited resources, stating that an increase in the number of immigrants would result in an increase in the population and strain available resources. The situation, he added, would lead to conflict over the resources, with all citizens clamoring for access.

He further argued that immigrants from certain regions would not fit within the American culture and social system because they had been subjected to a long period under a master and were predisposed to always wait for the master’s instructions. He also suggested the need to develop an exclusive and identifiable citizenry and to avoid the idea of turning the United States into an asylum for the oppressed from all nations.

The senator suggested that the United States should close its doors to new immigrants and instead focus on the people already within its borders, protecting the available resources and improving life for the people.

Senator Smith made this speech during debate over immigration restriction in the 1920s.  This was a time when many Americans had very nativist attitudes and were very much in favor of restricting immigration.  Smith's speech advocates restricting immigration and gives the major reasons why many wanted to take this course.

Smith's ideas can be summarized in this sentence:

I think we now have sufficient population in our country for us to shut the door and to breed up a pure, unadulterated American citizenship.

What he is saying is that there are enough immigrants in the United States to fulfill the needs of the country.  At the same time, he is saying that further immigration will dilute the country and its culture.  He is implying that the immigrants do not assimilate and become "true" Americans.  Therefore, he wants to "shut the door" and to "assimilate what we have."