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Party Discipline in the U.S. Senate and European Parliament

With voter identification of independent/non-affiliated voters on the rise, and more open primaries than ever, some have suggested that the end of the Democrats vs. Republicans system in the U.S. might be near. Political strategists and analysts regularly dissect the role of independents in electoral politics, insisting that “winning the independent vote” is key to success. At the same time, the rhetoric machines are pumping out ideas like “post-partisan,” and “governing towards the middle,” to explain people’s new views on politics.

At the same time, governing coalitions and traditional parties around Europe are falling fast in the polls, such as the Sarkozy presidency and Brown government, following the Czech government implosion, among many others. The run up to European Parliament elections, as mentioned last week, is featuring shakeups in nearly all of the 8 major coalitions, including several splits, one coalition that will likely cease to exist, and one new coalition, the European Conservatives.

Even as voters continue to support independent thinking, regular change in the political class, and a swinging ideological pendulum, however, it turns out that voting blocs tend to hang together with members exercising great discipline and regularity.

In both the US Congress context and the European Parliament context (as well as national parliaments), legislators have dual identities, first to their geographic constituency – US state/district or EU member state – and second to their ideological constituency, the party or coalition to which they belong. In addition, the individual ambitions and peculiarities of the members themselves complicate a clear mapping of interests. As such, putting together a cohesive voting bloc could easily become a real challenge.

In modern political thinking, a “cohesive” political party is one where a common set of ideas bind the members together, forming a bloc where voting interests are nearly uniform. For example, a party that consisted of U.S. Sens. Coburn, Cornyn and Shelby would be fairly cohesive.

However, a small cohesive party is not a party that can control the majority of a legislative house, and therefore party discipline is needed to bring together a group that can move legislation. These rules, trade-offs, bargains, with a topping of good old-fashioned coercion, make a relatively diverse group of interests work in unison.

Party discipline is particularly important for opposition groups and small coalition members, who maintain their influence by banding together, even if their interests are not particularly unified or cohesive.

Overall, party discipline in in the industrialized democracies is quite robust, even in relatively non-cohesive parties. Looking at the last ten years, discipline in voting by party in the U.S. has rarely fallen below 85 percent- only when the Democrats were at their weakest position in 2003-2004. In the last five years of the European Parliament, the trend has been for more variability in voting, but a still high overall average of between 83 and 90 percent.

While the trends are quite similar in overall voting clarity, one importance difference emerges between the Senate and the EP with regards to discipline. In the U.S. Senate, with two similarly-sized parties, the party in power tends to vote more similarly. Or conversely, the party that votes more similarly tends to be in power. The opposite seems to be true in the EP, with the ruling EPP-DE coalition struggling to match the voting unity of the smaller PES and Green parties. The two more volatile of the coalitions, the EPP-DE and ALDE – both alliances of two smaller pan-European parties – have in some years posted cohesion rates of 10 to 15 points lower than their rivals.

Overall, voting in modern democracies most commonly follow party lines. The ten to fifteen percent that fall outside are on issues where party coercion is not strong enough to maintain unity – often on issues where geographic interests dominate instead. On most contentious ideological issues, however, party identification is usually strong enough to hold the voting blocs together. These contentious or controversial votes are commonly the ones that build and break down parties, and therefore reinforce the importance of voting together.

In a couple of weeks we will see the final ideological reorganization of the European Parliament, as this process of party and coalition re-identification proceeds, quite similar to the way that primaries in the U.S. reorient the Republicans and Democrats. As the EP becomes a more mature body (this year makes 30 years), perhaps we will see a slow increase in voting clarity in the ruling parties as interests coalesce more coherently.

Renard Sexton is FiveThirtyEight’s international columnist and is based in Geneva, Switzerland. He can be contacted at