A real military conflict with China near China's front door is another thing. I doubt anyone with some common sense should call that an easy win.
The US has lost a lot of their soft power, beginning with the Iraq war, but exploding during the 45th Presidency. Their failure to convince Germany to drop North Stream 2 is the best example of that loss - particularly the gang of senators which thought it would be a good idea to threaten a harbor with sanctions [1] was perceived as extremely rude here. And a lot of nominal allies are highly skeptical of how reliable an ally the US themselves are, given the very high likelihood of the Republicans getting obstructionist power again either this November or in two years and the lack of decent candidates on the Democrat side.
[1] https://www.dw.com/en/nord-stream-2-us-senators-threaten-ger...
Though I think your point about obstructionist Republicans is well-founded and accurate and extremely accurate when it comes to domestic policies. With that being said I don't see a significant threat there in terms of US vs Russia or US vs China given that even when their vote was mostly inconsequential (i.e. they could try and virtue signal) most members of Congress, either Republican or Democrat, tended to vote to support US efforts against Russian aggression. The Senate was voting something like 99-1 to support Ukraine (don't quote me on the exact bill or bills) when they didn't really need to and could have tried to stick it to the Democrats as war mongering and followed in Trump's footsteps there.
For now it is, the interesting question is if the US voter class is ready to go to war again after the disasters of Iraq and Afghanistan - at least from an European POV it looks like wide swaths of the population do not want further overseas engagements and wish to reduce military spending in order to have more money available to fix pressing issues (crumbling infrastructure, healthcare, poverty, high energy prices). Additionally, while Trump himself is a very vocal opponent of China and I think Taiwan will be secured even if the Republicans win either of the next elections, it is nowhere near certain that Trump as a President will apply the same towards Russia or other enemies of the Western sphere - particularly not given strong indications of both himself and his close environment being influenced by Russia, both financially and via "kompromat" rumors (e.g. the "pee tape" allegation).
The main threat to the US is dissolution of the democracy, but I think even in that case there would still be strong (maybe even stronger) support for defense of Taiwan because it's an existential issue for the US. For China to invade Taiwan with impunity would dissolve all American credibility in the Pacific and the ramifications of that are significant. It also would kick the US out in terms of trade in favor of China, which would do damage to the US economy.
There certainly is some concern that China may try to engage in a fast, hard engagement with the US and that could cause the US to withdraw, but given the significance of maintaining economic and military interests it's likely that it would just lead to a swift, hard response and from there we're probably in a pretty destructive war. I don't think even Trump or someone similar could prevent a US response here.