Once upon a time, bipartisanship was not an illusion. It wasn't a panacea. It wasn't the key to getting most things done. But it was a good bet that if something needed doing, there would be at least some bipartisan support. In most cases, it just wasn't that hard to get. And that very fact meant that it wasn't a constant fetish, the way it is in Versailles nowadays. One reason for that was that the two parties had a non-trivial amount of ideological overlap, which is most consistently mapped by first dimension of the DW-Nominate scale. In those days--the first eight years of the 1960s, for example, here's what overlap in ideology in the US looked like:
In sharp contrast, here's the disappearing ideological overlap in the Senate the first eight years of the 2000s:
BTW, that's no overlap at all in the last two congresses before Obama took office. So the idea of getting "bipartisan support" as a prerequisite and a priority for any piece of legislation in this sort of political environment is nothing short of crazy. It is, in effect, a unilateral surrender of the majority to the minority, so long as the minority holds its ideological ground. And that is precisely what we have been seeing since Obama took office last January.
Dramatic as the above chart may be, it's not quite the same as seeing how that translates into an ideological overlap between the two parties--or the lack thereof. Which is where the charts at the top of this diary come in. They take the more abstract measure of ideological distance between the parties, and show what it means in terms of positioning the members as a whole in any one session. Larger, easier-to-read charts of the individual congresses can be found below.
But we don't have to content ourselves with looking at graphs, either. There's a third way we can grasp how things stand now compared to the past using the same underlying data. We can derive a fairly helpful rough metric--a single figure--to indicate the degree of bipartisan overlap. To do so, we simply count the total number of pairs of Democratic and Republican senators in which the Democrat is more conservative than the Republican. (This is an imperfect measure, since can have senators serving partial terms getting counted fractionally more than they should, but the imperfection is relatively minor.) If we do this, we come up with the following table: