In response to my post on Drew Westen’s latest, a few commenters took issue with a secondary point. (My primary point, that Westen mischaracterizes the partisanship of the mass public, attracted less dissent.) My secondary point was that, despite this stereotype that Democratic politicians are less disciplined than Republicans—more fractious, harder to coordinate, etc.—Democrats and Republicans in Congress have essentially equivalent levels of unity on roll call votes.

I honestly believe that most people who say that Democrats are less disciplined than Republicans do not know this fact about unity on roll call votes. That’s why I pointed it out.

Is roll call voting the entire story on party discipline?  Of course not. Let’s review some other evidence:

1. As I noted in the first post, Democrats and Republicans in the mass public vote for their party’s candidates at the same (high) rate. In presidential elections, party loyalty is approximately 90%. Here is data from the 2008 exit polls, for example.

2. Democrats are frequently portrayed as less ideologically cohesive than Republicans. Is this true in the mass public? It would sure look that way, given that there are more self-identified conservative Democrats than liberal Republicans. But as we also know, many people who identify as conservative support an ostensibly liberal position: government spending on various social programs. (See also this.  Or this.)  So I don’t know of an easy way to score this one for either party.  (I’m not prepared to take a dating site’s data as representative of the broader public.)

3. Networks of interest groups. As it turns out even though Democratic groups are supposed to be factionalized—e.g., unions, minorities, feminists, environmentalists, etc.—networks of Democrats groups are actually denser than GOP networks, as measured by patterns of endorsements and financial contributions.  See this earlier post featuring research by Matt Grossmann and Casey Dominguez.

Okay, but quit dodging the issue, Sides.  Westen specifically mentions parties “in Congress.”  One commenter noted that roll call votes were not a good measure of unity because leaders will only bring bills to the floor when the party is unified.  Yes, that happens.  Although not always. But let’s say, for the sake of argument, that party unity scores overestimate party discipline.

If they do, then how else might we measure discipline?

Some commenters seem to think that the Democratic caucus is more ideologically heterogeneous than the GOP’s.  One holds up taxes as an example.  The GOP will oppose tax increases in lock-step; but the Democrats do not support them at similar levels.  But contrary examples also emerge, notably the GOP’s attitude toward government spending—which, as in the mass public, is conflicted at best.  Which may explain why, despite Grover Norquist and “starve the beast” and “drown in it the bathtub” and so on, Republicans disagree about levels of government spending.   So you get stories like this.  Ultimately, I know of no systematic evidence—beyond what we could learn from roll call votes (and I don’t even know what those would show)—that the GOP’s congressional delegation is systematically more homogeneous across time and across issues.  I welcome someone to provide some evidence, as long as it’s more than just another anecdote.

Others think that the Democrats can’t coordinate on a message, which means, I guess, that if we put 5 Democrats on the Sunday talk shows, they’ll say 5 different things while Republicans will all recite the same scripture.  Do we have systematic evidence for this?  I don’t know of any either.  Again, I welcome it.  (UPDATE: See Sebastian’s comment to my earlier post, which he posted after I published this post.  He found a paper that examined the parties’ message discipline on the estate tax and found comparable levels of discipline.)

Ultimately, my sense is that the stereotype—Democrats are less disciplined than Republicans—is just not well-supported by systematic evidence that looks at all facets of parties, including voters, networks of groups, members of Congress, etc.  In other words, people are too sure of this generalization than the evidence can support.

Which brings me to a final, and perhaps more interesting question: where does this stereotype come from?  A colleague and I discussed this today.  The gist of our conversation:

1. Republicans were in the minority in Congress for many years, which meant that they had a greater incentive to stick together and oppose the Democrats.  This gave the misleading impression that they were all the same and Democrats were always infighting.

2. After the McGovern-Fraser primary reforms, it seems liked the Democrats were all helter-skelter, whereas the GOP always had a designated frontrunner.  I think 2008 and now 2012 should start to call that impression into question.  The GOP is certainly not immune from uncertainty about its nominee.

3. Republicans won more presidential elections than they lost for several recent decades.  Democrats blamed bad messaging, bad campaigns, bad candidates, etc.  There was a tendency to reach out, well, to people like Westen to tell them The Message that would enable them to win again. Needless to say, this overstated the role of messaging.

I welcome other explanations or reactions.

[Cross-posted at The Monkey Cage]

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John Sides is an associate professor of political science at George Washington University.