Paul Waldman at The American Prospect puts it very well here:
The rational politician is one who knows how to maneuver around—and exploit—his constituents' irrationality. At times though, that irrationality can enslave the politician to positions that are bad for his party, even as they make perfect sense to him at a particular moment.
In this particular instance, Waldman is describing why it is rational for someone like Mitch McConnell to describe the ACA as an ongoing disaster that must be repealed, even as the law improves the lives of millions of his KY voters. The political lay of the land in health care has been like this for decades: rich doctors and poor whites opposed to reform, no matter how terrible our health care system works. The difference maker now is that the ACA has passed into law and millions are newly insured, making continued opposition dangerous for the GOP in the long run. Eventually they will look like cranks whining about a loss.
But more generally, the big change in the rational calculus of a GOP politician is the rise of super PACs, especially those funded by lunatics and libertarian True Believers. In the old days on an issue like immigration, crossing the Chamber of Commerce might be popular with voters, but it would lead to a lack of campaign cash. Now, post Citizens United (the case), the Club for Growth, Citizens United(the super PAC), and other Tea Bag coffers are there to "save" the day. A GOPer can now run against the Chamber and still be well funded.
For instance, the Republican party needs to pass immigration reform to show Hispanic voters it isn't hostile to them, particularly in the face of a growing Hispanic population. But if you're a Republican congressman in a conservative, majority-white district, what's good for the national party would be deeply irrational for you. So you condemn illegal aliens and pledge to fight against comprehensive reform, then get safely reelected, along with a couple hundred of your colleagues who do the same thing. And the reform that your party needs never comes to pass. In other words, politicians can only be as rational as their constituents allow.
For Waldman, the analysis stops there: the rational GOPer wins his race but hurts the party nationally, presumably Waldman means it hurts in the Presidential election.
But changing demographics and the slow but inevitable invasion of facts will eventually hurt in local races, as well. Both forces push the general election voter further away from the candidate that appeals to the Tea Party base, i.e., the people who vote in GOP primaries.
In more and more general elections, at every level, only a reality-based candidate who refuses to pander (via dog-whistle) to bigots of various stripes will be able to win. But at the same time, nothing I can see is likely to change the fact that in GOP primaries, both the money and the votes are strongly aligned with pure crazy, i.e., reality denying haters.
Slowly but surely, in more and more places, winning a GOP primary will mean--by definition--that you cannot win the general election that follows. Eventually, the GOP will go the way of the Whigs.