Halperin points to Richard Ben Cramer's 1988 book What It Takes as providing the justification for judging candidates based upon campaign skills. Kevin Drum thinks the narrative is older:

Perhaps it's this: in the older books, the fact that presidential candidates had to survive an ungodly gauntlet of scrutiny and rubber chicken banquets was reported as a fact of life, but it was (again, to my recollection) mostly reported as an unfortunate fact of life. As in, "How unfortunate that some of the people best suited to be president will never have a chance because they aren't suited to the preposterous rigors of modern campaigning." Maybe after 20 years of this, Cramer provided the press for the first time with a rationalization for its part in this destruction test: don't think of it as unfortunate, think of it as necessary. By making a mountain out of every molehill, reporters are actually providing a stern test that eliminates weaklings who shouldn't be trusted to have their fingers on the button.

Perhaps. But regardless of whether this is true, it's merely a rationalization. Contemporary campaigns may be even more grueling than they were a few decades ago -- thanks to modern technology, longer primary seasons, and a bigger press corps -- but I doubt that What It Takes is really responsible for the media's current fascination with personality and horserace journalism. That's always been there.

I think Drum lets the press corps off too easily. It's not just that they've come to cheer on the horserace. They are now the jockeys, whipping the candidates into jumping over more and more ridiculous hurdles.

The inanity of modern campaigns has much to do with the foibles of members of the press. After all, they've helped create the modern media environment through their biases. And I don't mean political or ideological biases, though they have plenty of those. But much more decisive are their biases toward laziness and stupidity.

Consider one story from Marc Ambinder's article in the December 2007 issue of the Atlantic Monthly, "Teacher and Apprentice":

Here, Obama's novelty worked against him. The national press corps places tremendous importance on consistency with an established narrative. Lacking a basis to judge Obama's neophyte foreign-policy views, reporters were much more willing than they otherwise might have been to accept the Clinton campaign's charge that Obama's answer was naive. They weren't nearly as willing to accept the countercharge from the Obama campaign that Clinton had herself flip-flopped in answering the question (earlier in the year, while criticizing Bush's recalcitrance about meeting with rogue leaders, she had expressed practically the same sentiment as Obama), because such a slip-up didn't track with the emerging campaign narrative of Clinton as disciplined and savvy. Nor could Obama's campaign deploy, as Clinton's did, an army of surrogates to flood the airwaves and drive home the point. In August, Obama told a reporter that under no circumstances would he use a nuclear weapon to destroy terrorist bunkers in Afghanistan or Pakistan. The Clinton campaign again pressed the charges of inexperience. A year earlier, as it turned out, Clinton had said essentially the same thing as Obama in response to Bush administration posturing about nuclear weapons. But reporters largely ignored this fact, because it wasn't in character for Clinton to mess up. (p 62) [emphasis added]

Here we see the press corps completely swallowed up by gamesmanship. The story went beyond merely ignoring the substance of the policies in question. It went beyond mere speculation on campaign savvy. The story was about how reporters became an echo chamber for the Clinton campaign's lies.

And this is when members of the press aren't actively trying to make up the narrative. Consider how much worse it is that we let newsreaders with enormous egos 'moderate' the debates. Matthew Yglesias explains the tedium of Enemy #1: Tim Russert. The whole charade is maddening -- and a large part of why I ignore almost all campaign reporting. The only worse things are 1) that the median voter has even lower standards than reporters and 2) the low quality of candidates themselves.