There’s a second reason the “Etch-a-Sketch” moment Eric Fehrnstrom just created for the Romney campaign is significant beyond the fact that it reinforced a negative stereotype about Mitt’s lack of “a core:” he really can’t just “shake it up and restart” his campaign, or adopt a different agenda, or forget about the promises he’s made to conservatives, even if–and that’s a big if–he wants to.

As Dave Weigel noted in the Jan/Feb issue of the Monthly, long before voters begin weighing in, Romney capitulated to Tea Party pressure and made the most important promise to the Right any candidate could make:

Mitt Romney, the presumptive front-runner who Tea Partiers are happiest to trash on the record, [is a] good test case for what the movement can extract from candidates before the polls open. He was cagey during the debt fight right up until June 29, when he endorsed the “Cut, Cap, and Balance” pledge. FreedomWorks, Dick Armey’s group, had endorsed the pledge on June 28. It was a radical set of promises. Developed by the conservative Republican Study Committee, endorsed immediately by big Tea Party groups, it committed the signer to reduce nondefense discretionary spending to 2008 levels, cap spending so that it falls below 20 percent of GDP by 2021, and get ratified a new version of the Balanced Budget Amendment that prohibits tax increases unless two-thirds of the Congress agrees to them.

And now, cutting and capping is Romney’s default answer to thorny debate questions about the budget.

And as Jonathan Bernstein explained in the same issue of the Monthly, presidential candidates are almost always stuck with their campaign promises; that’s why we all remember it so vividly on the rare occasions they don’t:

[A]s you listen to Mitt Romney and the rest of the Republicans as they debate and make speeches and release policy papers, don’t assume that it’s all meaningless, empty rhetoric that will be dropped once the campaign is over and governing begins. Don’t assume, either, that since the Republican nominee will no doubt move (rhetorically) to the center after clinching the nomination, specific pledges made in the primary season will be left behind—remember the story of George W. Bush and tax cuts. The truth is that careful observation of the candidates really can tell us a good deal of what they’ll do—and what they’ll be like—as president.

And finally, also in the Jan/Feb Monthly, Thomas Mann and Norm Ornstein made a strong case that even if he is elected president, Romney would not have the power to reverse his party’s rightward direction even if–and again, that’s a big if–he wants to:

A President Romney would be in a poor position upon taking office to change the course outlined in his campaign. He is already suspected as an infidel by many Republican activists. His fiscal policy would almost certainly be ambitious, one not unlike the budget resolution written by Representative Paul Ryan and passed by House Republicans. Indeed, this is the course Romney has taken with his professed economic plan, released in early November. If Romney tried to dilute his own proposal, he would be met at the beginning of his presidency with a full-scale revolt on his hands from his own party, both in and out of Congress.

So Mitt really is in a triple bind: he has a reputation–which his own staff is reinforcing–for being willing to abandon the few principles he has at the drop of a hat; yet all that does is make voters from both parties mistrust him; and in any event, he’s stuck with the promises he’s made and the party he seeks to lead.