Unfortunately war does tend to have a way of rallying both sides together. Tribalism tends to just need an "other" group to demonize.

A 9/11 scale attack would make it pretty easy for supporters of both parties to turn their sights on whoever they think attacked us.

That's certainly true in general but I can't think of an attack that was preceded by such a visible effort to make the country more vulnerable. There would be some rallying effect, but it might be smaller and shorter lived than usual especially if the response is bungled. Then again, things might have become so cultish that reality completely stops mattering and trump skates by once again without any responsibility.

Reality isn't simple enough to peg responsibility on any one person.

Unless you go with the "shit rolls down hill" approach and ultimate land blame for everything on the person in charge, it'd be extremely hard to find a clear line between 6 weeks of Trump's presidency and an attack on the scale of 9/11.

Maybe you could tie it back to connections with his first term, but any attack on that scale takes years to plan, not weeks, and has plenty of facets to consider when it comes to what motivated the attackers to actually do it.