It's become extremely common for conservatives to assert that the crash of the financial system in 2008 almost single-handedly enabled Barack Obama to win the presidential election. See, for instance, this:

“We were crushed by circumstance,” communications director Jill Hazelbaker said after McCain’s speech. “The economic crisis was a pivotal point in this race.”

Or this (from Charles Krauthammer):

It was rejection of the Bush administration, a weariness of war, and the rejection of an administration at a time of economic collapse. It was nothing more.”

I've seen iterations of this argument countless times. It's a seductive explanation for conservatives -- it explains for them how such a radical candidate could win a solid majority in a country they deem totally hostile to his program. It absolves conservatism from Obama's triumph, which can be explained away as a pure fluke.

How true is the explanation? Not very, though there is a related truth that I'll explain. Obama did pull away from McCain in September. But numerous things other than the financial crisis occurred. First, and most importantly, you had the inevitable wearing-away of the GOP convention bounce. Second, you had a series of disastrous media interviews by Sarah Palin that transformed her into a massive drag on the Republican ticket. Third, you had three presidential debates in which the public by overwhelming margins believed that Obama crushed McCain.

The financial crisis is not what doomed McCain -- he was trailing in the pools for months, with the totally predictable exception of the short period following his convention -- and was doomed to lose. What killed McCain is that the economy in general was terrible:

The upshot is that things were even weaker in the 2006 to 2009 time frame than was previously recognized. In 2008, for example, the Commerce Department had earlier estimated 0.4 percent growth for the year; it now estimates that GDP was unchanged.
While the Great Recession began in 2008, you'll recall, it was a relatively mild downturn in the early months of the year and only turned severe in the fall.
That year, commercial real estate investment, inventories, and imports were bigger drains on economic activity than earlier estimated.
Overall, for the 2006 to 2009 period, GDP decreased at an average annual rate of 0.2 percent, compared to a zero percent average annual change previously estimated. We already knew that the late years of the last decade were pretty disastrous for the American economy; the revised data put an exclamation point on that conclusion.

Conservatives don't want to admit that the economy leading up to the crisis of 2008 was terrible, because that damns the Bush supply-side experiment. They want to blame the financial crisis, which is easier to paint as a black swan fluke event that does not reflect upon conservative policies. But the truth is that the economy cooked McCain's goose long before the financial system crashed.