What racial group is the most pervalent in America?

Well, why didn't you state it that way, in the first instance?
'Caucasian', is still not entirely accurate, but it is a step in the correct direction. 'Uralic', would probably be a better description, except in its present context, it could easily be confused with the Finno-Ugric peoples, proper.

With modern genetic evidence, and to the best of my current knowledge, it is becoming quite clear that the ancestral tribes which inhabit Europe and adjoining areas, originated in or near the Ural mountain region, not in the Caucasus mountains, as was previously thought.