Are Republican Groups Using Foreign Money to Fund US Elections?

I've touched on this before, but it is very curious that it's raising so few eyebrows outside of liberal circles.  Yes, it's true that this massive influx of corporate cash into the political process is primarily benefiting Republicans right now.  But since the Citizens United case in the Supreme Court is allowing unlimited corporate donations to political races, exactly what is being done to ensure that foreign corporate money isn't steering some of the decision making?  Even if it is being done--this time--to help the GOP, aren't there any conservatives out there troubled by the possibility that foreign companies and even foreign countries could be helping decide elections?