"If the economy hadn't tanked, McCain would've won"

[... but first, I'd like to try to dispel a myth: the U.S. economy did not suddenly fall off a cliff in September. It had been on a downward trajectory for quite a while - this downward trajectory became impossible to ignore in September. Keen observers - Paul Krugman and Atrios among them - had been shouting at the top of their lungs for more than a year about the coming crash. Both Sen. Obama and Sen. Clinton recognized at least some of the economic ills last spring.]

Onward.
A common talking-point - particularly among Republican apologists - is that McCain would've won - or at least not lost so badly - had the economy not tanked in September. Who knows, they may be right...

BUT... this serves only to highlight a weakness of the Republican brand: a Republican candidate has no chance to win if the economy is in tatters. Why not? Presumably because Republicans don't know how to fix a broken economy!
There are two other plausible alternative explanations - other than the weakness of the Republican brand... and they are self-reinforcing:
Alternative #1: When the economy tanks, the incumbent Party gets the blame. [The Republicans broke the economy!]
Alternative #2: McCain's economic policy, such at it was, simply extended W's policy - which led to disaster. [McCain proposes to perpetuate the policies that broke the economy!]

Sen. Obama also found himself faced with the very same economic crisis, and with the very same two months left till the election. Why was he not at a disadvantage?
Well - there are Alternatives #1 & #2, above: Obama's not a Republican... and his economic policy - outlined fairly clearly on his campaign website - is not a continuation of W's follies.

Other than that?
Obama seemed calm in response to crisis. McCain seemed unhinged.

A few of the talking heads have noted this, and suggested that it wasn't the economic crisis per se that did in McCain - but rather McCain's response to the crisis that doomed him: first pronouncing the fundamentals of the economy strong, then suspending his campaign, then unsupsending his campaign, then throwing out solution #1, then solution #2... then solution #n - without any apparent serious thought behind any of 'em.

This 'chicken-lacking-head' behavior contrasted markedly with Sen. Obama's universally-acknowledged calmness in the face of crisis.

So, yes - without the economic crisis the electorate may not have seen Sen. McCain's inability to respond calmly to a crisis... and in that sense perhaps it was the economic crisis that doomed his candidacy: the crisis revealed for all the world to see just how poorly McCain handled the challenge it presented!

Have a nice day!