The elections can be skewed with different kind of manipulation (social media, troll farms, electronic ballot hacks) and the popular candidate can still win.

It's fair to assume Putin would try to influence the elections in trump's favor as he would have been beneficial for him.

The Ukraine invasion would have been much easier if trump had been relelected, finished pulling out of nato and kept the more isolationist policy that was in effect during trump.

IMO trump was also doing a lot of damage to the US and it's position on the world stage which is also beneficial for Putin.