We're two nights into the Democratic National Convention, and the themes could not be more distinct from those championed at the RNC last week. Whereas the RNC heavily emphasized the role of personal initiative in economic success, the DNC's speakers have focused on the many barriers that keep success away from even determined, hard-working Americans.

It's a sign, the New Republic's Alec MacGillis argues, that the parties are getting more ideologically coherent — that is, they have more sharply defined, and sharply distinct, viewpoints than they once did. Democrats and Republicans are now, he writes, "ideologically coherent to the point where they make even Europe's parliamentary parties look muddled by comparison."

But is this showing up in actual congressional votes? To find out, I looked at Keith Poole and Howard Rosenthal's "DW-NOMINATE" database, which uses congressional votes to measure the ideological position of members of the House and Senate.

Steve Smith, a political scientist at Washington University in St. Louis, and Sarah Binder, who's at George Washington, computed the standard deviation — which measures variation in scores — for each chamber and each year to see how widely scores ranged within each party, and graciously passed the results on to me. If the standard deviation is bigger, the party is less unified. If it's smaller, the party is more unified. So how do the standard deviation figures change from Reconstruction to today? Here's the Senate:

In the 1920s, Republicans held a wide range of views on economic issues, with both progressives like Robert La Follette — who favored railroad nationalization and was endorsed by the Socialist Party of America when he ran for president — and free-market conservatives like Calvin Coolidge within the GOP fold. Then both parties grew more unified during the Depression, and then more diverse again after World War II. Since the 1960s, both parties have grown more unified, but Senate Democrats more so. So what does the House look like?
The parties diverge less here, but the story of parties — especially Democrats — growing more ideologically diverse after World War II and then more homogenous from the 1960s on out is still in view. Steve Smith — the WashU professor who gave me his data on this — cautions that the metrics are less useful the farther back you go, so let's just look at data since 1947. The blue lines are Democrats, red Republicans, dashed lines the House, and solid lines the Senate:
In recent years, the trend has been for Democrats — especially in the Senate – to grow more unified as the Republicans largely stay the same. A large part of this is likely the exit of Southern Democrats from the coalition, meaning many conservatives left the Democratic fold, but in the House the Democratic convergence continued well into the 1990s and 2000s. All of which suggests that, contrary to popular belief, Democrats aren't actually worse at party discipline, and aren't more ideologically varied, than Republicans. If anything, in recent years they're grown more ideologically coherent than the Republicans.

Poole and Rosenthal also put out "party unity" scores which measure how frequently members vote with their parties on key issues. Interestingly, these numbers show much more similar behavior between the parties. These numbers measures the percentage of votes that members of each party voted with their party on:

In you case you missed it: in 2009, House Democrats and Republicans voted with their parties an average of 93 and 91 percent of the time, respectively. In the Senate, it's 93 percent and 89 percent. The parties, in short, are extremely unified, to an unprecedented degree.