I'm not talking about an overseas terrorist attack where a few people are killed. I'm speaking of an attack on the homeland where tens of people die.... I think that would encourage Americans(and thus politicians) to come together.

Thats now what the model is saying. At all.

Do you really think if a 9/11-esque attack happened tomorrow that Republicans and Republicans-calling-themselves-Independents would support Obama the way Democrats/Independents supported Bush? I think that's impossible to believe given how they've behaved for the entirety of Obama's presidency. There would be no rallying around the flag; Republicans would be falling over one anther to say "see we told you, his failure to say the words 'radical Islam' caused this."

It wouldn't even matter if we had proof of a specific actor, they would blame Russia, Iran, ISIS, Cuba, and whoever else they could shoehorn into the mix. It'd be Obama's fault for waging the "War on Christmas" instead of saying "radical Islam" 15x a day. It'd be Obama's fault for actually trying to prevent a dangerous nation from obtaining a nuclear weapon, unlike how his predecessor just allowed North Korea to march towards one. It'd be Obama's fault for going along with the sequestration cuts they wanted, even though that probably wouldn't prevent any large-scale attack if one actually happened. It'd be Obama's fault for "placating the gays" by allowing women and LGBT enlisteds to take on larger roles in the armed forces. It'd be Obama's fault for "withdrawing when victory was at hand" in Iraq. Obama's fault, Obama's fault, Obama's fault.

If Republicans turned a major terrorist attack into a political fight, they would turn off the vast majority of Americans. Many Republican voters would not stand for it. Republicans wouldn't suddenly bend over in the fights over domestic issues, but I think they would be more cooperative and less divisive in their rhetoric. Hopefully the president would follow suit.

unless said terrorist attack is basically assassinating the president - in which case you'd have republicans celebrating everywhere

If by Republicans you mean Muslims all around the world, I tend to agree.

They have turned off the vast majority of Americans, hence their massively-underwater favorability rating. It doesn't matter what Republicans do anymore -- Democrats hate it, "true Independents" hate it, Republicans-who-call-themselves-Independents don't like it but vote for them anyway, and Republicans like it anyway. This is the new normal after Bush's presidency, and after the way they've conducted themselves during Obama's presidency.

Republican voters wouldn't like it but they'd still vote Republican, just like they do now.