The Senate side of the Capitol, seen in 2017. (Mark Wilson/Getty Images)

Forty-four former United States senators wrote an open letter Monday to their colleagues now serving in that body, asking that the Senate stand in defense of democracy. Perhaps in part because the signatories were both Democrats and Republicans, the specific call to action was a bit vague, but the implication was obvious: The Senate has a role to play in constraining the worst impulses of President Trump.

Skimming the list of signatories, something quickly jumps out. Many of those who added their names to the list were not exactly the most ideologically extreme members of their respective caucuses when they served.

We can visualize that, thanks to data compiled for each Congress by the VoteView project, now housed at the University of California at Los Angeles. That data includes a measure of ideology on a numeric scale that can be used to track individual and caucus partisanship over time.

So here’s a complicated graph with some of that data. We’ll explain in a second.

(Philip Bump/The Washington Post)

Notice the dark red and blue lines. Those are the average ideological scores for the letter’s signatories in each Congress. The lighter-colored lines are the average scores for each party’s members overall. (For this analysis, we excluded independents caucusing with either party.) The blue and red shaded areas indicate the range of values in each party. Where they overlap — where some Republicans were more liberal than some Democrats or vice-versa — we’ve colored in light purple.

Two things jump out. The first is that the Democratic signatories were about as ideologically partisan as the Democrats on the whole, while the Republicans (from about the 95th Congress on) were more moderate than the caucus.

The second, and perhaps more important, thing to note is that the solid areas of color went from having a lot of purple to having open space between the two parties.

We can visualize that erosion of the middle ground by plotting every senator’s ideology for the past 70 years. In the 80th Congress (1947 to 1949), the shape is like a bell curve. By the 100th Congress, the two parties had mostly separated.

(Philip Bump/The Washington Post)

That was also about the peak of when the signatories served. The gap between the ideologies of the senators from either party had been widening over time, but by the 111th Congress or so, the gap began to grow much wider fairly quickly.

(Philip Bump/The Washington Post)

You can see why on the long chart showing every Congress: The addition of much more ideologically extreme Republicans pulled the average to the more conservative end of the spectrum.

This is telling in itself. Congress has grown more polarized along with our politics. Trump’s election and his presidency have depended on the strength of partisan support from Republicans far more than an effort to reach out to Democrats and independents. Republican members of the House and Senate feel a pull from their party’s supporters to embrace Trump, a pull that has led to members of the Senate giving Trump a pass on things that would in the past have seemed impossible.

Those 44 senators (three-quarters of whom are Democrats) are calling on the Senate to hold a tougher line. But the Senates in which they served — and Senates like those — no longer exist.