Although I'm a citizen and resident of the US, I can't help but smirk when government officials are outraged and shocked that another nation state is trying to

Although I'm a citizen and resident of the US, I can't help but smirk when government officials are outraged and shocked that another nation state is trying to influence our electoral process. Question for someone more informed than I: With the Citizens United ruling, what's to stop another country from pumping money into US elections?