There's an old economics joke that goes something like this: 

A physicist, a chemist and an economist are stranded on an island, with nothing to eat. A can of soup washes ashore. The physicist says, "Let's smash the can open with a rock." The chemist says, "Let's build a fire and heat the can first." The economist says, "Let's assume that we have a can-opener ... "

Moderate Republicans endorsing Romney have been doing something similar. They all work off of a similar premise. "Let's assume we have a Democratic Senate," they begin.

You see this in David Brooks's endorsement. The vision of Romney's presidency begins with: "Romney would probably be faced with a Democratic Senate." You see this in David Frum's endorsement. "I don't expect" Republicans to retake the Senate, he writes, and then he goes further: "A Romney presidency likely means that the congressional GOP will lose seats in 2014, as they deserve." So Frum's vision of a Romney presidency assumes a Democratic Senate now and a more Democratic Congress later. His colleague, Justin Green, also endorses Romney by saying, "The next president, be it a reelected Barack Obama or a newly elected Mitt Romney, will face a Republican House and a Democratic Senate."

All of these endorsements are dealing with the same problem: They want to endorse Mitt Romney, the moderate governor of Massachusetts. But over the past few years, we've mainly seen Mitt Romney, the "severely conservative" champion of the Tea Party. The only way to ensure we don't get that guy is to give Democrats the Senate. 

But that's not actually something that can be assured. Democrats might lose the Senate tomorrow. If they don't, they might lose it in 2014. Or 2016, when President Romney is reelected atop a booming economy, and right before the unexpected retirement of one of the liberal members of the Supreme Court. What happens then? Are the endorsements void?

It's a strange kind of endorsement that only works as long as the presidential candidate being endorsed isn't able to govern alongside members of his own party. More to the point, it's a self-nullifying kind of endorsement.

If you think Romney would govern successfully alongside a Democratic Senate, then you probably think his approval ratings will be high going into the midterms, or at least going into his second term. If that happens, then voters will reward his success by electing more members of his party to Congress (and recall that Romney, in Massachusetts, was energetic if ineffectual in campaigns to elect more Republicans to the legislature). Given the Senate map in 2014, that will mean full Republican control of Congress. So if the endorsements are correct, then the conditions they assume will quickly vanish.