The issue of foreign influence in US Presidential elections has been and continues to be a hot button.
Sometimes the focus on election hacking is on hacking the ballot box, but while this is possible, it would be very hard to do that on a national scale, so it is unlikely that this is the tactic that they would take. However, since we know that Russia attempted to penetrate election systems in all 50 states during the 2016 elections, we should not rule this out completely.
Whether the foreign powers want to help or hurt a particular candidate (and there are likely some of each), there are many things they could do.
Obviously, they could hack the emails and other systems of candidates and release embarrassing emails. They could also hack candidates personal phones and computers in addition to the campaign’s systems.
More likely, these powers will launch disinformation campaigns. The number of emails that I get on a daily basis that are designed to inflame or contain outright lies is amazing an will only increase as we get closer to the election. Same thing with social media. Whether people will disregard these campaigns is not clear. It seems that people tend to accept spam that they agree with and reject spam that they disagree with as opposed to treating it all with a whole lot of skepticism.
While it is illegal, foreign governments have been injecting money into campaigns of candidates that they like. This is done via proxies who can contribute, so figuring out who is a shill for, say, China, might be hard.
Remember also that hacking elections is a time honored tradition. While the techniques have gotten better, hacking elections is not new. One source says that the US interfered with 81 foreign elections (that we know about) since 1946.
The bigger issue is that people THINK that the elections are rigged and do not vote at all. If this happens, the bad guys win.
Voters need to be on the alert for all kinds of tricks that a foreign OR DOMESTIC actor might try. Smart voters will reduce the impact of the bad actor’s work. And you must vote.