[B]oth parties have grown more extreme in recent years. Congressional Republicans certainly have. Congressional Democrats tend to be more moderate, relatively speaking.
The perception that the Democrats haven't shifted radically left in recent years is due to libertarianism agreeing with what that shift represents more than disagreeing with it. And frankly, the evidence A. Barton Hinkle cites shows how the whole country has indeed shifted left. Not completely, obviously, but shift left it has, and that libertarians can't see that tells you more about libertarianism than libertarianism tells you about libertarianism.
It's not that Republicans have become more extreme. It's that the country's shift to the left has isolated them. And Democrat positions are only "moderate" in the sense that they are now more widely shared. It's the growing isolation of Republican conservatism in the face of these which only makes it seem extreme. It would be more accurate to say that Republican positions have become anachronistic, not extreme.
Hence much of the recent evidence cited by Hinkle which demonstrates where Americans are united today is of the "shift-left" variety, including:
62% now believe in gay marriage when for generations the vast majority of Americans did not, and for millennia human beings did not, and anti-sodomy laws still dotted the land up to 2003;
73% now favor utopian pipe dreams of "alternative energy" when it was coal, oil and nuclear which made America the industrial powerhouse of the world;
73% now unsurprisingly favor euthanasia just 44 years after the Supreme Court made it legal to murder unborn children;
83% favor "medical marijuana" despite the evidence of its risks for human health and well-being;
85% want to let the Dreamers stay;
90% favor universal background checks for weapons purchases;
83% disavow "extremist bigotry" under the influence of multiculturalist indoctrination in American public schools.
And libertarians are pretty much on board with these things, along with most Democrats. That's why all the action is in the Republican Party. The war for its soul continues to animate the present time. The Democrat soul already belongs to the devil.