(Erika Rich/AP)

The point of the piece is that the left hadn’t been particularly interested in the Republican presidential primary until now. They didn’t think Michele Bachmann could win and, secretly, deep down, they didn’t think Mitt Romney was so bad. But Perry potentially could win, and for reasons both of culture and policy, liberals think he could be very, very bad. Which probably suits the White House just fine.

Democrats didn’t lose 63 House seats in 2010 because the American people referred Republicans to them. That was part of it, of course, but at least as important was the so-called “enthusiasm gap”: the fact that Democratic voters didn’t prefer Democrats strongly enough to come out and vote in their usual numbers.

The way you measure the enthusiasm gap in an election is you take the partisan breakdown we see in polls -- in general, slightly more Americans self-identify as Democrats than as Republicans -- and compare it to the partisan breakdown we see at the ballot box. In 2008, for instance, the electorate identified as 7 percent more Democratic than Republican. In the polls leading up to the 2010 election, the electorate identified as 5 percent more Democratic. In the election itself, Democrats and Republicans turned out in the same numbers -- an enthusiasm gap, compared to the 2008 election, of 7 points. And as Nate Silver has documented, a six-point swing is likely to mean the difference in 25 House races and, making matters worse for Democrats, the enthusiasm gap was largest in the swing states.

The expectation was that the enthusiasm gap would decline in the 2012 election, when Barack Obama was back on the ballot, and so too was a Republican presidential candidate. But with Obama’s poll numbers dropping even among self-identified Democrats and liberals, you could imagine the enthusiasm gap dropping by less than the White House had hoped. That would be particularly true if Republicans nominate Romney, who can appeals to Democrat culturally -- he did win elections in Massachusetts, after all -- and manages to project the sense that he is, at heart, a competent technocrat who is not nearly as extreme as his party.

Perry, by contrast, is much more extreme than his party. His book recommends, among other things, repealing the 16th and 17th Amendments and unwinding Social Security, Medicaid and Medicare. He’s much more interested in playing the cowboy-vs-cityboy culture card. And you can see the effects taking hold. My colleague Ruth Marcus, who is about as even-keeled and moderate as they come, was clearly a bit shocked reading Perry’s book. The subtitle of the Texan’s manifesto is ‘Our Fight to Save America from Washington,’ but Marcus writes that “reading it summons the image of another, urgent fight: saving America from Rick Perry.”

This is the sort of countermobilization that the Obama campaign is unlikely to see if they run against Romney but can almost bet on if they run against Perry. Perry excites people who agree with him and scares people who don’t. That does imply some offsetting effect on the Republican side, as he may turn out more GOP voters than Romney would. But given the Republican Party’s desire to get Obama out of office, it’s hard to imagine too many conservatives staying home under any circumstances.

But it’s not that hard to imagine some Democrats staying home. That is, unless the other name on the ballot is Perry, or someone of similar ideological leanings. Fear is as effective in getting people to vote as hope. And if the Obama campaign isn’t able to recapture the hope that turned its voters out in the 2008 campaign, it may find a candidate like Perry useful for filling in the gap.