"government health care is less efficient than private insurance" (never mind the data, much less the health care choices of much of the rest of the world).
The danger comes when fighting over a "matter of principle" outweighs all other factors, including and especially any real-world consequences. People who are rationally, economically afraid to go the doctor until absolutely necessary are more likely to be sick for longer, or die earlier -- but that doesn't matter to politicians who "know" that universal health care is just "bad." Children who aren't being taught in schools what science actually is are more likely to lag in scientific understanding behind those in countries that do -- and science gives us technology, and all the industrial products and productivity thereof. Pretending that teenagers just won't have sex because you tell them "don't do it" means you wind up with more kids who will have difficulty pursuing their dreams via college or other means, because they are suddenly saddled with a responsibility they never asked for.
And most currently, die-hard intransigence against tax increases or reform of virtually any kind, especially concerning the rich and/or large corporations, has not only led to hostage-taking against the long-term unemployed, but also directly threatened -- and damaged -- the full faith and credit of the United States debt for the first time in US history. The stock markets are currently reacting not only to the actual S&P downgrade, but to the "give us what we want or the world economy gets it" tactics of Tea Party Republicans and the associated consequence that the US government is unlikely to take any new spending actions to aid an already-feeble economy further damaged by the antics of the Republicans.
The danger of such absolute moral certainty is a total lack of caring or compassion for actual Americans hurt by such policies. In fact, one could go further, cynically questioning whether such moral certainties are even genuinely held, or merely convenient bludgeons. Mitch McConnell, Republican Senate Minority leader, is on record as stating that Republicans' "number one priority" is not jobs, nor the economy, nor a balanced budget, nor unions or abortion or "traditional marriage" or any one of the things Republicans have been talking about, but defeating President Obama. This, to the expense of all else, including the common good of the very people they are supposed to be representing. Certainly the call for "fiscal responsibility" and balanced budgets so loudly trumpeted by the Tea Party of late has been mysteriously absent during any Republican administration in recent history.