Political scientists Matthew Grossmann and David Hopkins think the Democratic and Republican parties really are different, and in a series of papers, they're trying to prove it.

In "Policymaking in Red and Blue," Grossmann and Hopkins state their conclusion plainly: "the Republican Party is dominated by ideologues who are committed to small-government principles, while Democrats represent a coalition of social groups seeking public policies that favor their particular interests."

Policymaking has a liberal bias

Let's pause here. The word "ideologue" is a technical term within political science but an insult within American politics. There is nothing wrong with approaching politics ideologically — and that's particularly true when you compare it to the major alternatives, which are approaching it transactionally or as a pure partisan. Nevertheless, if I keep writing this piece using the word ideologue it will sound like I'm just insulting Republicans over and over again. From here on out I'm going to use the less precise, but also less loaded, "philosopher."

BOehner: "We ought to be judged on how many laws we repeal."

So let's restate: Democrats are more focused on making policy to appease their various interest groups and Republicans are more focused on proving their commitment to the small-government philosophy that unites their base.

As Speaker John Boehner put it when he was asked about the slow pace of lawmaking in his House, "we should not be judged on how many new laws we create. We ought to be judged on how many laws we repeal."

I came to Grossmann and Hopkins' papers skeptically. Most efforts to outline the differences between the two parties end up being reasons that the author prefers one party to the other, or prove to be descriptions of the parties right now that will change as soon as the White House flips. But Grossmann and Hopkins have amassed some impressive evidence.

1) There are more conservatives than liberals but more Democrats than Republicans

This is an old finding in American politics, but a powerful one. More Americans consistently identify as conservative than liberal. At the same time more Americans consistently identify as Democrats than as Republicans.

On its face, this presents a puzzle: how can conservatism be the more popular ideology even as the Democrats are the more popular party?

Grossmann and Hopkins disagree. They see this not as a puzzle about American politics but as an explanation for why it works the way it does. They note that 73 percent of Republican voters say they're conservative but only 42 percent of Democratic voters say they're liberal. And they note that while voters tend to agree with Republicans on the philosophical questions in American politics (should government be smaller?) they tend to agree with Democrats on the policy questions in American politics (like should Social Security be smaller?).

73 percent of Republican voters say they're conservative but only 42 percent of Democratic voters say they're liberal

The Republican Party, in other words, has a very good reason to base itself around philosophical conservatism, while the Democratic Party has a very good reason to base itself around policy deliverables. And so the Republican Party bases itself around philosophical conservatism and the Democratic Party bases itself around policy deliverables.

The question, of course, is how to test this: what would you ask Democrats and Republicans to test whether one side was there for the philosophy and the other side was there for the policy? Luckily, pollsters have more or less solved this problem.

2) Republicans prefer purity, Democrats prefer compromise

Since at least 2007, the Pew Research Center has asked Democrats and Republicans whether they prefer politicians who stick to their principles or politicians who compromise. This is a clever way of testing voters' interest in passing policy, as the American political system famously requires compromise to get anything done.

The chart above shows the results: Democrats consistently prefer politicians who compromise and Republicans consistently prefer politicians who stick to their principles.

"Republicans have been consistent in their elevation of principle over moderation, regardless of which party is in power"

What's remarkable is that held true even when Republicans controlled the White House. "Though they voiced strong disapproval of Bush, Democrats still expressed a preference for compromise in government — a tendency that has carried over to the Obama era," write Grossmann and Hopkins. "Republicans have been consistent in their elevation of principle over moderation, regardless of which party is in power."

That is...extraordinary. Even when a Republican president was facing a Democratic Congress, Republicans did not choose the answer that would have helped their president get more done. And even when a Republican president was facing a Democratic Congress, Democrats did not choose the answer that would have stiffened their party's spine against passing Bush's bills. I would have bet money against surveys showing this kind of stability between Democratic and Republican administrations. This is a difference between the two parties that runs deep.

3) Democrats are under more pressure from interest groups to pass policy

Another difference between the Democratic and Republican parties is that Democrats answer to more interest groups than Republicans.

Grossmann and Hopkins assemble studies showing that Democratic delegates at both national and state conventions report more organization memberships than Republican delegates, suggesting that Democratic conventions are the site of more organized interest group activity than Republican conventions. They also note a study showing that more interest groups make endorsements in Democratic primaries than in Republican primaries.

The graphic above is perhaps the most persuasive evidence of the density of the Democratic interest-group ecosystem: it connects interest groups that endorsed more than one of the same candidate or bill in the 2001-2002 Congress and the 2002 midterm election. So, if the AFL-CIO and the Sierra Club both endorsed Senator Mary Landrieu for reelection and they also both endorsed No Child Left Behind, they get a line. The more shared endorsements between two groups, the thicker the line connecting them; the more total connections any individual group has to other groups, the larger the circle they get.

there are more organized groups asking Democrats for policy than asking Republicans for policy

You can see the results. The ecosystem of interest groups making endorsements on the Democratic side is both larger and more interconnected than on the Republican side; there are more organized groups asking Democrats for policy than asking Republicans for policy.

But Democratic interest groups aren't just more numerous; they're also more persistent. "The Democratic Party contains strong links between its electoral and legislative coalitions...The diverse groups that come together to support the same candidates also ally when it comes to passing bills in Congress," write Grossmann and Hopkins. "The Republican Party lacks similar ties between its electoral and legislative coalitions, mostly because few of its groups regularly join coalitions to support or oppose legislation."

Which isn't to say that the Republican Party doesn't have plenty of interest groups demanding its fealty: the National Rifle Association, the Chamber of Commerce, and National Right to Life all hold enormous sway. Indeed, the relative paucity of interest groups on the right might make the ones that do exist stronger, as Republicans have fewer opportunities to play them off each other. But these numbers help explain why Democratic elected officials feel more pressure to deliver new laws than Republicans.

4) Policymaking has a liberal bias

Democratic presidents talk more about policy, propose more specific policy ideas, and pass more significant pieces of legislation. The numbers are stark. Since 1945, Democratic presidents have put forward 39 percent more policy proposals than Republican presidents, and 62 percent more domestic policy proposals.

"There is a good reason for this asymmetry," write Grossmann and Hopkins. "Democrats and liberals are more likely to focus on policymaking because any change that occurs is much more likely to be liberal than conservative. New policies usually expand the scope of government responsibility, funding, or regulation. There are occasional conservative policy successes as well, but they are less frequent and are usually accompanied by expansion of government responsibility in other areas."

The chart above codes significant policy changes by whether they expand or contract the "scope of government regulation, funding, or responsibility." Policy changes turned out to be more than three times as likely to expand the scope of government than to contract it. This is often true even when Republicans are signing the laws.

President George W. Bush is a good example. He passed a series of tax cuts which conservatives mostly liked. But his other major domestic accomplishments — No Child Left Behind and Medicare Part D — sharply expanded the role of the federal government in education and health care, and today they're used as evidence that Bush wasn't really a conservative president.

The cleanest way to shrink the size of government is to repeal laws and regulations. But it doesn't happen very often. In the American political system, Grossmann says, "it's hard to pass anything, but it's particularly hard to repeal a law that already exists." Systematic analyses show it's rare for laws to be repealed wholesale. "That creates perpetual disappointment among the Republican base," Grossmann continues. "They correctly perceive that their party does not succeed in enacting their professed ideology."

As such, gridlock is often the best small-government conservatives can hope for. And so they're more comfortable with it than Democrats.

The parties act differently because they are different

This data only takes you so far. "Conservatism" is more than just a preference for small government. Democrats are only somewhat more likely to introduce new legislation than Republicans. As Grossmann told me in an interview, "these are differences in degrees that are based on a difference in kind between the party coalitions."

But they're a reminder that American politics is fundamentally rational. Republicans are uncompromising because compromise tends to expand the scope of government. Democrats are willing to make deep concessions because policy moves in a generally liberal direction. Republicans have a clearer message about government because their message about government is fundamentally popular. Democrats talk more about policy because what they have to say about policy is fundamentally popular.

Republicans are uncompromising because compromise tends to expand the scope of government

The data also explains why Democratic and Republicans have so much trouble understanding each other. Democrats tend to project their preference for policymaking onto the Republican Party — and then respond with anger and confusion when Republicans don't seem interested in making a deal. Republicans tend to assume the Democratic Party is more ideological than it is, and so see various policy initiatives as part of an ideological effort to remake America along more socialistic lines.

My main objection to Grossmann and Hopkins' argument is that the relationship between the coalition and "the party" isn't unidirectional. Politicians can change the minds of voters just as surely as voters can change the minds of politicians. What counts as "liberal" has a lot to do with what various influential liberals end up embracing; consider, for example, the way the individual mandate went from something liberals loathed to something they supported over the last 15 years.

It's also worth remembering that these are relative differences between the two parties. Republicans have their policy wonks. Democrats have their ideologues. This data shows the parties are different — not that they never overlap.

Still, there's much that this data does illuminate. I've often heard liberals wonder why there's no Democratic analogue to the Tea Party. I've often heard conservatives complain that their party doesn't spend enough time coming up with serious policy solutions for issues like health care. And, to be sure, there are some liberals trying to popularize Tea Party-like tactics and some conservatives trying to come up with sweeping new health reforms. But it's hard for these initiatives to succeed. There's a tendency to imagine the parties as mirror images of each other, and so to believe they can easily follow the other's strategies. But they can't. The parties are good at different things because they really are different.