One reader at TPM pointed this out:

I think there is one point to remember when Republicans keep saying that they are so surprised that core groups in the Obama electoral coalition, like African Americans, young voters, etc., were able to match or even exceed their 2008 turnout: Republicans did some pretty unbelievable, disrespectful and frankly unconscionable things to this President that JT’s cites: shouting “You Lie!” to him during the middle of his State of the Union address (something that was frankly never contemplated to be done to Clinton or Bush, despite rapid opposition), challenging his birthplace and religion, or Governor Brewer pointing her finger in his face on the tarmac, much of which was repeated nightly on places like Fox News.

Regardless of whether these things were done because of the President’s race (and I think that a pretty convincing argument could be made that a lot of what happened was at least partially due to his race), the fact of the matter is that Republicans who engaged in this type of behavior honestly shouldn’t be surprised now that there was some consequence to their actions, and by this I mean that the President’s supporters, who felt and understood this disrespect, would be extra-motivated to support him in response to these antics.

I would agree with that, but would say it’s not the whole story. I think a good deal of it was also the awareness that, despite any and all of the left’s disappointments about Obama not being lefty enough, we were strongly aware that there was a huge difference between Obama and Romney.

This was our bane in the 2000 election—too many people, especially the 2.74% who voted for Nader, felt that there was little or no difference between Gore and Bush, only to be horrified at how wrong indeed that was. Our budget surplus wrecked and exploding deficits like none we had seen before, rampant partisanism and legislative bulldozing from the right, two massive land wars in Asia—I could go on, but you probably remember the highlights.

These voters realized that Gore would not have instituted full-on class warfare and while the surplus may have evaporated, it would not have changed to trillion-dollar deficits. That even if Gore had let 9/11 happen, he would have been reserved in Afghanistan and never would have gone into Iraq. That Gore would never have been the simple-minded sock puppet Bush wound up being.

This realization of differences only became more sharply defined in 2008, when McCain started kowtowing to extremists, and then chose Sarah Palin, who was Bush on steroids, in all the worst ways. We had seen moderate Supreme Court Justice O’Connor replaced with an ideological soulmate of scumbag Antonin Scalia, and the Chief Justice replaced with a young staunch conservative, and realized that had things gone differently, the court could have transformed into a body that would never have sunk to the rank political depths of Bush v. Gore.

Romney only continued to sharpen the distinction. Like McCain, he was a flip-flopper bowing to the extremists, a rich, privileged white man—but this time one who represented the worst of the Wall Street excesses that we recoiled from so violently in 2007 onward. And he chose as a running mate a poster boy for the dismantling of Social Security and Medicare. That, with four septuagenarians on the bench.

Yeah, I would say there was motivation from that direction as well.