Securing the border is a MANDATORY duty put on the federal govt by the Constitution. It's not a suggestion or a recommendation but an affirmative duty assigned specifically to the federal govt with clear, unambiguous language. I would argue the Constitution does not allow for the shifting of that mandate to private citizens.
The United States shall guarantee to every State in this Union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.
-- Article 4 Section 4