Bonica and Sen trace politicization through the state and federal judiciary, showing how partisan processes used to select judges create politically polarized courts, particularly at the federal level.
They use a massive database of lawyers' and judges' contributions to political candidates between 1979 and 2012 to illustrate the effects of partisanship on the judiciary. Those donations allow them to calculate ideology scores of jurists, based on the simple logic that “someone contributing to a liberal/conservative candidate is more likely to be liberal/conservative herself.”
They then look at the distribution of those ideology scores for subsets of lawyers who go on to become judges at the state or federal level. They find that at the federal court level in particular there's a significant partisan divide between liberal and conservative judges. There are many conservative judges, in other words, and many liberal judges. But not very many in between.
If federal judges were nonpartisan automatons blindly applying the rule of law you might expect that chart to be flattened out, with more judges in the center and fewer at the poles. But the chart looks the way it does because judges are, in fact, living, breathing partisan creatures, just like the rest of us.
There's another interesting wrinkle in these numbers. If you look not at judges but rather at all attorneys in the U.S. (e.g., the pool potential judges are selected from), you see that the attorney pool is considerably more liberal and less conservative.
If you were blindly plucking a random sample of attorneys out of this pool to fill federal judgeships, you'd expect partisan distribution of judges to match the partisan distribution of attorneys. But that's clearly not the case.
The reason, Bonica and Sen explain, is that the selection of judges is itself a highly partisan process. And conservative politicians, it seems, have done a much better job than liberals of funneling ideologically like-minded judges to the upper reaches of the judiciary — particularly when you consider that there are more ideologically liberal lawyers in the pool of potential candidates.
Bonica and Sen credit the efforts of groups like the Federalist Society, which has deep roots in elite legal institutions like Harvard and Yale, with cultivating generations of conservative lawyers more likely to go on to the judiciary than their liberal peers.
For instance, the study found that conservative graduates of Harvard, Yale and the University of Chicago law schools were 12 times as likely as their liberal peers to attain a seat on the Federal Circuit Courts.
Gorsuch himself is the product of successful partisan efforts to shape the Supreme Court, after the Senate blocked President Barack Obama's nominee for the seat until President Trump took office 11 months after it became vacant.
In the end, it shouldn't be surprising that an inherently partisan selection process yields an inherently partisan court. As legal scholar Lee Epstein told the New York Times in 2012, “I think voters — not to mention senators, presidents and the judges themselves — understand that there’s a difference between judges appointed by Republican and Democratic presidents. If they didn’t, we wouldn’t have confirmation battles.”