Tomorrow, the so-called Gang of 10 will meet at the White House for what’s expected to be a lengthy discussion over debt reduction. The goal will be an agreement that resembles a compromise, with the intention of crafting a plan that garner acceptance from a Republican House, Democratic Senate, and Democratic president.

There’s no great secret about what will serve as the greatest barrier to success: the GOP doesn’t believe in compromise. House Speaker John Boehner isn’t even comfortable saying the word out loud.

It’s easy to forget how central the notion of compromise is to the entire American political process. In our system, there’s an expectation that members of a party’s caucus will compromise amongst themselves, leading to another compromise with the opposition caucus, leading to another compromise with the other legislative chamber, leading to another compromise with the executive branch.

Congressional Republicans, unrivaled in their discipline and ideological coherence, have established a set of demands. Asked what they’re willing to compromise on, GOP leaders tend to tell the truth: “Nothing.” They reject the very idea of accepting concessions, and assume the country stands behind their inflexibility.

It’s come to define Republican politics in the Obama era: all compromise is bad in all instances. This includes policymaking, but it also includes electoral considerations and GOP primaries.

Nate Silver did a nice job this week explaining why this has come to be: “The Republican Party is dependent, to an extent unprecedented in recent political history, on a single ideological group. That group, of course, is conservatives… [T]he terms ‘Republican’ and ‘conservative’ are growing closer and closer to being synonyms.” Nate supported the argument with this chart, showing “the ideological disposition of those people who voted Republican for the House of Representatives in the elections of 1984 through 2010.”

The party has always had at least a plurality of self-described conservatives voting for GOP candidates, but now it’s no longer close. Conservatives simply dominate. The total is up, you’ll notice, more than 20 points over the last 20 years.

Add in the fact that Republicans, for whatever reason, have begun announcing their opposition to their own ideas, the moment Democrats express a degree of support, and we’re left with a party that won’t, and probably can’t, give an inch on anything.

And what of Dems? Self-described liberals are growing as a percentage of the Democratic electorate, but self-identified moderates are still the plurality, and the combined total of moderates and conservative Democrats represent a majority.

To drive the larger point home, Kevin Drum flagged this chart, showing Democratic and Republican voters’ willingness to support lawmakers who “compromise to get things done,” as oppose to those who stick to principles “no matter what.”

The differences are stark. Democrats are pragmatic and want accomplishments; Republicans are dogmatic and want ideological purity.

If the debt-reduction talks breakdown tomorrow — or more likely, the participants end up with a lousy deal that makes GOP leaders happy — keep these charts in mind.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.