In itself? No. It depends largely on WHY they want the US to remain majority white and Christian. Assuming they are Christians themselves, if they believe other races are not made in the image of God to the same extent that whites are, then I believe that would fall under my previous definition of believing other races are sub-human.
If they simply want to preserve the "city on a hill" idea that many of the first immigrants wanted to establish here, and that this is best accomplished by keeping the US predominantly white and Christian, then I see no reason to call that racist, any more than folks wanting to keep Japan predominantly Japanese. Cultures will naturally want to preserve what makes them unique.
Once it crosses the line into having a superiority complex on the basis of your race alone, especially if it then crosses into seeking the subjugation or elimination of other races due to their supposed inferiority, that is when it becomes racism, in my opinion.