The anti-Reaganesque knock on McCain is that his instincts are not libertarian; he courts  establishment culture and craves its approval.  And he violates Reagan’s famed 11th Commandment, which prohibits Republicans from speaking ill of each other.  But, generally, McCain can pull it off.

The peril of Reagan comparisons, either implicit or explicit, can be greater for some candidates. Comparing oneself to Reagan is less likely to elevate one’s own Reaganesque attributes than to invite invidious and memorable comparisons from one’s opponents and the media—courting the conclusion that one is barely like Reagan at all.

Mitt Romney ran into this problem early on. To conservatives at a National Review dinner, he allowed that he had changed his position on abortion, and that he had not always been a Reagan conservative. (In fact, in 1994, he asserted, proudly, that he had been an independent during the Reagan years.)

Even Ronald Reagan, Romney averred, had not always been a Reagan conservative. He too, had changed his position on abortion. Reagan “learned with experience,” Romney said.

So—Romney compared himself to Reagan by qualifying Reagan’s "Reagan" essence. That’s hard for Reagan admirers to swallow. Romney also missed the historical point: Mr. Reagan became pro-life against the consensus of his party, and did so years before he was elected president.

Two weeks ago, Romney was honored at something called the Frontiers of Freedom Ronald Reagan Gala in Virginia. Many of its sponsors were campaign supporters, but no matter. He began by saying it was an “honor” to be “associated in a small way with the legacy of President Reagan who fought tirelessly to lift the nation.” But he was fairly circumspect and, to my ears, doesn’t talk about Reagan as much as he used to.

Of course, Romney can legitimately claim Reagan as a model and a political hero today. But he is not a Reagan Republican:  when it counted—during Reagan’s presidency—he rejected that label. The more he claims that identity now, the more he has to fight for it.

On a broader level, Republicans who try to use Reagan to guide their thinking on policy fall victim to an historical fallacy at the heart of any attempt to use Reagan to guide policy today. The war on Islamic terrorism is simply not the Cold War, but Republicans often invoke the analogy when they want to challenge Democratic policies on defense and national security. The more one tries to model an election on a president who left office nearly 20 years ago, the fewer corners one can  actually see around.

And the politics have changed, too: Reagan-era battles about the size of government have put Republicans on the defensive today. Voters now see Republicans as the profligate party, not the Democrats.

Moreover, the qualities that contributed to Reagan’s global leadership skills might not be relevant today either. George W. Bush is principled, uncurious and resolute; even some of his closest friends believe he is convinced his policies are sanctioned by God.  But over the past few years, several of the Republicans now running for president say that a more subtle approach to the world might have helped.