There's no way to know how compromised U.S. elections are

To what extent and to what end those exploits could be used to actually affect the outcome of national elections are still open questions. Many experts still believe that it’s unlikely for such efforts to meaningfully affect presidential elections, and the chances of actual votes being changed are still very low. The decentralized and incoherent nature of American voting systems actually plays out in favor of security, because of the enormous effort it would require to hack enough individual systems to change national outcomes. And there’s not yet any evidence that Russian hackers, or those of other origins, have altered even a single vote; that itself may be instructive.

But well-targeted local and state campaigns can move some needles politically. Gumming up the works on Election Day, causing errors leading to long lines and lower turnout, forcing volunteers to use backup systems, and casting doubt on individual registrations are other possible uses of hacking. There is already plenty of evidence that these problems do affect turnout and perhaps electoral outcomes. Additionally, the sheer likelihood of identity theft at the ballot could—and should—give voters pause on Election Day. And if those kinds of attacks are—or were—executed successfully, they’d be by design very difficult to trace.