Harris would have a very awkward time as president. She's not very popular, even among the Democrats. Those who are farther left typically distrust her for having been a prosecutor (I don't personally share this opinion, but it is one that I've seen before).
While China is a very real threat, I do not think that they can actually replace the US in its current role. A lot of this is due to geography. China's only coast is highly contested (the US, Taiwan, Japan, Australia, the Philippines, Vietnam, Indonesia, and others all have stakes in the western and southern Pacific Ocean). Unless they can secure loyalty and cooperation from all of these countries, which would be quite difficult to do since many of them despise or distrust China, it'll always be difficult for China to project power.
The US has two mostly uncontested coastlines. This makes it easy for the US Navy to go pretty much anywhere in the world. Even if China did somehow secure its coastline, it'd have great difficulty in reaching the Atlantic Ocean, meaning that it would face great difficulty in being more than a regional power.
This is not getting into the various problems that China faces. China, much like Japan, lacks sufficient birth rate to replace its aging population. Young people in China face many of the same difficulties as their counterparts in the US, like homes being far too expensive for most people to afford (and this also clashes with the cultural expectation of owning a home in order to get married). There's also the fact that many families only have one child. This means that military losses could have pretty serious domestic repercussions.
This is not to say that China is weak, but to point out that they do suffer from some pretty serious limitations and internal problems. If China attempts to unseat US influence in the Pacific, they may very well succeed. It's not certain that the US will defend Taiwan, as the US is not actually obligated to. A war between China and the US risks escalating to the point of going nuclear, which will devastate both countries, as well as many others. However, China actually supplanting the US is quite unlikely for the simple fact of geography. Even if the US suffers some sort of serious internal issue (Yellowstone supervolcano, major economic downturn, etc), China cannot actually take the US's place.