As America’s two major political parties have evolved in the direction of philosophical purity over the past half century — with the Democrats emerging as the home of ideological progressivism and the Republicans as the font of ideological conservatism — it has become common for each to accuse the other of extremism.

Republicans call the Democrats strident socialists eager to bring about the End of Freedom in America, while Democrats accuse the Republicans of waging a War on Women, African-Americans, Hispanics, and just about anyone else who isn’t a Wealthy White Man. The vacuous centrism of inside-the-Beltway conventional wisdom then reinforces the pox-on-both-your-houses narrative, treating both sides as equally to blame for every failure to reach consensus and Get Things Done.

The reality is far less fair and balanced.

Over the past six years, Barack Obama has shown himself quite willing to compromise with Republicans, while Republicans have demonstrated over and over again that they have no interest in cutting deals with the president. (Number of Republicans in the House of Representatives to vote for President Obama’s 2009 stimulus bill? Zero. Number of House Republicans to vote for the Affordable Care Act? Zero. And so on.)

Whether this is because of the GOP’s principled opposition to Obama’s policies, or its Machiavellian conviction that the president is hurt more than the opposition party by inaction in Washington, or (more likely) some combination of the two, the end result is the same: The Democrats prove themselves to be a pragmatic, centrist party, while the Republicans consistently demonstrate no-holds-barred ideological stridency.

We saw further examples this past weekend, at the Iowa Freedom Summit, where a long list of GOP presidential hopefuls spoke to adoring crowds in Des Moines.

Texas Sen. Ted Cruz advocated an ideological litmus test: “Every candidate’s going to come in front of you and say, ‘I’m the most conservative guy to ever live.'” But “talk is cheap,” he insisted. “Show me where you stood up and fought.”

Now imagine a liberal presidential candidate taunting fellow Democrats, daring them to demonstrate their progressivism and willingness to stand up and fight for it.

Unlikely.

Wisconsin Gov. Scott Walker, meanwhile, plans to build a national campaign “on his record of defying teachers’ unions.”

Now imagine a Democrat building a national campaign on a record of defying police unions.

It wouldn’t happen.

Then there was rabble-rousing neurosurgeon Ben Carson, who promised that he would dismantle ObamaCare “even if it worked.”

Now imagine a Democrat showing an equal disdain for pragmatism by promising to prop up a government program “even if it doesn’t work.”

I don’t think so.

On and on it goes, with the GOP’s would-be presidential candidates competing to stake out the ideologically purest, most unambiguously right-wing position. An analogous scramble to the left just doesn’t happen among the Democrats — or at least it hasn’t happened since the time of the Reagan administration.

The question is why.

The answer has nothing to do with the machinations of party leaders or anything else that originates in Washington. On the contrary, the stance of each party reflects above all else the ideological makeup of its most loyal voters. And the fact is that in the United States, right-wing Republicans outnumber left-wing Democrats by a significant margin.

As the Pew Research Center showed last summer in an important report on political polarization, 22 percent of the general public identify as conservative (either socially or economically), while just 15 percent think of themselves as liberal.

Those are the relative sizes of each party’s ideological base.

The gap increases to 27 percent conservative and 17 percent liberal when highlighting registered voters. And it increases even further — to 36 percent conservative and 21 percent liberal — among the most “politically engaged” Americans.

Electorally speaking, Republicans are being pulled to the right by public opinion much more powerfully than Democrats are being pulled to the left.

This is one significant reason why the RealClearPolitics cumulative average of polls currently shows just 16 percent of Democrats supporting left-wing candidates (Elizabeth Warren or Bernie Sanders), while nearly double that percentage of Republicans (30 percent) favor right-wing options (Ben Carson, Mike Huckabee, Scott Walker, Ted Cruz, or Rick Perry).

It’s also one important reason why Hillary Clinton — a candidate only a right-wing Republican could consider a radical lefty — currently enjoys 61 percent support among Democrats, while the more moderate Republicans (Mitt Romney, Jeb Bush, Chris Christie, and Marco Rubio) receive a comparatively lukewarm combined total of 43 percent. (I’ve left Rand Paul, with 6.8 percent, out of both camps because his positions defy tidy ideological categorization.)

The GOP is a party increasingly being steered by its most stridently ideological voters. Which is one reason (among many others) why I won’t be voting for a Republican anytime soon.