Can anyone hack the U.S government?
Sure. After all, the Russians did it and when there are some clever Russians out there that can do it, there surely will be some clever Chinese or other individuals that can do it as well. In fact, I hope they will hack the current administration or at least those systems that hold Trump's tax returns and then leak them anywhere else but to Wiki Leaks as they most likely won't publish them given their preferences. On the other hand, this would be THE opportunity to rehabilitate themselves from the suspicion of taking sides, so there might be a chance that they will publish it - anyway, before leaking copies to them, they should keep a second copy to leak somewhere else, just in case.