A big part of the reason for the continuing focus–angry as it makes people like Brendan Nyhan–on the Etch-a-Sketch incident is that it provides the dying campaigns of Mitt Romney’s Republican opponents such a graphic metaphor for the criticism of Mitt they so signally failed to make effectively during the campaign when it mattered.

Today at FiveThirtyEight Nate Silver asks why that was the case–why no one seemed able to hit that big fat bullseye on Mitt Romney’s back that so many of us thought would be the political death of him? Nate explores a lot of theories, including inadequate voter information on Romney’s background and the army of Establishment figures willing to validate his ideological reliability. But the big factor seems to be that his rivals had their own problems:

Tim Pawlenty famously flubbed a line about Mr. Romney’s health care bill during an early debate. Newt Gingrich and his “super PAC” attacked Mr. Romney on his tenure at Bain Capital, but was much more reluctant to go after Mr. Romney’s inconsistencies — possibly because Mr. Gingrich has some apostasies of his own.

Ron Paul, who has perhaps the most consistent voting record of any presidential candidate in memory, has rarely gone after Mr. Romney directly. Jon M. Huntsman Jr. was more willing to, but he was a minor player during most of the campaign, and he lacked the budget to give the slick commercials he had produced on Mr. Romney’s shifts a wide airing on television.

Two rivals, Rick Santorum and Rick Perry, were potentially in a stronger position to attack Mr. Romney on these grounds. But Mr. Romney’s campaign successfully executed the same bold strategy against them, shifting the conversation to highlight their own departures from conservatism.

I think what we’ve all been missing, however, is exactly why all these right-wing challengers to Romney–these teeth-grinding ideologues so full of contempt for RINOs and hatred for Democrats and the media–had their “own departures from conservatism.”

It may be as simple as this: even as the GOP has become significantly more conservative in just the last few years, the definition of “true conservative” has shifted just as far and as fast. And in the context of a competition in which no one other than Ron Paul–who for his own mysterious reasons has refused to criticize Romney–has a record that can pass muster under today’s definitions of conservative orthodoxy, Mitt’s relatively high levels of past heresy have simply gotten lost. To put it another way: the fact that Mitt was considered dangerously moderate, even liberal, by the standards of 2005 conservatism has been submerged in the general suspicion of a presidential field whose “true conservatism” in 2005 flunks 2012 standards of ideological consistency.

I’m sure this makes Newt Gingrich–once the walking, talking human definition of movement conservatism–completely crazy. His advocacy of perfectly orthodox conservative policy positions on the environment and health care are now being treated as precisely equivalent to Mitt’s heresies–even though Romney did a whole lot more than make a speech or two on subjects like climate change and mandate-based health insurance expansion–while obscuring Mitt’s vastly more controvesial advocacy of baby-killing, sodomite-coddling and “radical environmentalism.” And how about Rick Santorum? Always, always considered a conservative fanatic by Democrats, the kind of man liberals frightened their children with, he’s right down there in the compromising, RINO muck with Mitt because he voted for earmarks and endorsed Arlen Specter. Both he and Rick Perry–who began this campaign as the personification of vein-popping Tea Party rage–achieved negative moral equivalence with Romney for supporting famous Bush Administration policy initiatives (No Child Left Behind and Medicare Rx Drugs for Santorum, tolerance of the children of undocumented workers for Perry). But nowadays supporting the domestic policies of the man who was the universal champion of movement conservatives in 2000 is a terrible, black mark on your record, no different than Romney’s frankly liberal mid-2000s Republicanism.

So ironically, yesterday’s conservative firebrands look like Etch-a-Sketchers, too, when measured against the ever-more-conservative zeitgeist, and have lost the standing to go after yesterday’s moderate, Mitt Romney.

No wonder Santorum and Gingrich are brandishing a children’s toy, shaking it at voters in last-chance primaries and implicitly saying: You don’t understand! He’s as bad as Obama!

That’s probably true by very recent standards of conservatism. But by today’s more rigorous standards, everybody’s suspect, so why not go with the guy who seems to have the money and the general-election poll standings? It’s a question Mitt Romney’s rivals haven’t been able to answer. And that’s why he’s going to be the nominee.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.