This article notes: “Immigration is a net plus for Social Security. Because immigrants are younger, on average, than other Americans, they will contribute to Social Security for decades. Ironically, undocumented workers contribute even more. They are prohibited from receiving Social Security, even when they can prove that they have contributed. The Social Security Administration has estimated that undocumented workers pay billions of dollars to these programs each year but earn no benefits.”
Thus, fewer immigrants will mean less money for Social Security.