So, for starters, I am describing people by where they live, not how many live where.

Yes, of course, many more people live in major urban areas than other wise.

But it is still true that by area, there are many areas in WA and OR that are very highly Republican... presumably you have noticed the dramatic cultural difference between two of such of those areas.

Again, go actually look into the Greater Idaho project, a lot of these people are very serious about ... seceeding their counties from Dem dominant WA and OR state governments, to join ID's Rep dominant government.

This is a concerted political campaign that has been going on for over a decade and has gotten quite a lot of popular support in those counties via county level referendums.

.......

Further... the other point I am making is that the red areas are often quite extreme in their affinity for firearms.

It... does not take too many armed terrorists, militia cells, patriots, whatever you want to call them, to effectively terrorize or act as an insurgency in an area... especially if the official police are generally amenable to your movement... cops tend to be right wing authoritarians.

If you have ever lived in Portland or Seattle, I am sure you have observed the regular flooding in of non locals, from environs, whenever some kind major sport event, or public event occurs, or even just generally during work hours, good times to go shopping or clubbing or w/e.

Now just imagine that including some domestic terrorists.

........

This almost certainly would not play out as simply as the red and blue areas just... becoming their own nation states and then raising their own formal militaries to do battle with each other, where the red areas would lose simply by having less people or less industrial base.

No, civil wars, balkanization, sectarian violence, insurgency, occupation... is way way way more complicated than that.

Maybe go read up on how that went/is going in Iraq and Afghanistan or Syria or Ukraine... for some recently case studies on how complicated this can get.