Our astute political readership is well aware that the United States Senate has been divided into three classes since the beginning of the Constitutional Republic. That’s because, with a six-year term for each senator, only one-third of the Senate is elected every two years. Senators were elected by the state legislatures until the ratification of the 17th Amendment in 1913, of course, but the classes were maintained with the electoral reform, and as new states were added to the Union, the principle of “one-third every two years” has been continuous. While the U.S. House of Representatives is (theoretically) “refreshed” in its entirety by the People at each election, the Senate is much more stable, since two-thirds of the Senate membership is immune from popular uprising in any given election. Passions are given a chance to cool, or to reconstitute, before the next election rolls around.

One might think that each class is a random sample of the states, and that the combination of states every two years is irrelevant to the election outcome. And were you to think that, you would be wrong.

Instead, by accumulation of circumstance and partisan realignment in certain states and regions, each class of senators now has a somewhat different party tilt. Each class of Senate seats is shown below, with the percent of the vote won in each state by Democrat Barack Obama in 2008. (We could classify the states via a long-term presidential vote average, but in a way, this would be unrepresentative of current demographic trends. We believe using the most recent election for president is the best rough measure of party standing for now.)

Class I senators, last elected in 2006 and up next in 2012, are the BLUE CLASS. This is the most Democratic-leaning group of states, having cast a mean 53.2% and a median 55% of the vote for Obama in 2008. Fully 22 of the 33 states in this category gave their Electoral Votes to Obama. Maybe not surprisingly, this Senate class produced a turnover of the Senate to the Democratic Party in 2006 when almost every close contest fell into the Democratic column.

Compiled by Joe Figueroa, U.Va. Center for Politics

Class II senators, last elected in 2008 and up next in 2014, are the RED CLASS. This is the most Republican grouping of states. John McCain won a majority of these states (18 of 33), and Barack Obama secured just 48.3% (mean) and 47% (median). Given this Mars-like complexion, it is remarkable that the Democrats got a net gain of eight Senate seats in 2008—testimony to the powerful Democratic undercurrents in a strongly anti-Bush, anti-GOP election year. These results remind us that a tsunami can wash out the color in any Senate class of states. Of course, looking to 2014, it might also suggest that GOP Senate gains are more than a theoretical possibility, especially if this is President Obama’s “sixth-year itch” election.

Compiled by Joe Figueroa, U.Va. Center for Politics

Class III senators, last elected in 2004, and up this year and again in 2016, are the PURPLE CLASS. President Obama won 50.4% (mean) or 51% (median), and 19 of these 34 states—almost precisely equidistant from his showings in the BLUE CLASS and the RED CLASS. (Note that we are excluding all special Senate elections from the tally since the data base of the analysis is the permanent Senate seat class structure.)

Compiled by Joe Figueroa, U.Va. Center for Politics

We might expect the Purple Class III states to be the most competitive grouping, but this doesn’t mean they are immune from a strong trend. Republicans certainly hope this is true come November, and the early poll results suggest they are right. But Class III states do tend to shift with the prevailing winds. Early in the 2010 cycle, it looked like Democrats would hold their own or maybe even add a seat or two in November. Now of course, Republicans are expected to pick up multiple seats, perhaps 7 or so—a substantial advance but not yet enough for a Senate takeover.

In any election year, it will be far more important to examine the individual seats on the ballot, the number and condition of incumbents running again, the open seat contests, and the electoral conditions prevailing at the time of the balloting. Still, the notion of Senate classes is an intriguing one, and a good starting point for analysis in any election cycle.