President Trump and Senate Democrats are steadfastly opposed to one another over judicial nominees. Sens. Debbie Stabenow, D-Mich., and Gary Peters, D-Mich., are now threatening to "blue-slip" Trump's nomination of Joan Larsen to the 6th Circuit Court of Appeals. Other senators are making similar threats. With Republicans poised to reform the blue slip, now seems an appropriate time to discuss what it is and how it works in practice.

The blue slip is an informal tool that a federal nominee's two "home state senators" can use to support or oppose his or her nomination. For example, when the president nominates someone to be a lower federal court judge, the Senate majority leader refers that nomination to the Senate Judiciary Committee. The Judiciary Committee chairman then sends a blue slip of paper to the two home state senators (regardless of whether they are in the majority or minority party) asking them to rate the nominee. Senators have three options: return the blue slip to the Committee Chair with a positive recommendation, a negative recommendation, or refuse to return it at all (which is essentially a negative recommendation).

A negative blue slip is difficult to overcome. During the George W. Bush presidency, the chair held confirmation hearings in only 17.65 percent of nominations in which a senator returned a negative blue slip.

Senators argue that the blue slip forces presidents to consult with them before nominating people to positions that directly influence their constituents. But the data suggest senators use blue slips for a different purpose. Three features of the blue slip should inform the current debate over whether Senate Republicans should remove or reform it.

First, ideology drives senators' use of the blue slip. And not even solid legal qualifications can protect circuit court nominees against it. In a recent paper, my coauthors and I examined the conditions under which senators blue-slip judicial nominees. This was no easy task. The Senate does not usually release these data and newspaper accounts are spotty. Nevertheless, with some help, we were able to collect data on all blue slips between the 107th and 110th Congresses (2001-2008).

The data reveal that senators are much more likely to blue slip nominees who are ideologically opposite them. That is, liberal senators were much more likely to blue slip President Bush's judicial nominees than moderate or conservative senators. Perhaps more importantly, though, the data show that even stellar qualifications cannot save nominees to the circuit courts of appeals from the negative effects of ideological distance. (Strong qualifications still protect ideologically distant district court nominees from senators.) Ideologically distant senators were just as likely to blue-slip highly qualified circuit court nominees as they were poorly qualified circuit court nominees. Put plainly, senators have taken an aggressive role in blocking highly qualified nominees who would otherwise make significant—but opposing—policy even though those nominees are clearly qualified for the job. The blue slip has become an ideological contrivance.

The blue slip began as an information-gathering tool used to detect filibusters. As one important study points out, its first recorded use was in 1913—four years before the Senate adopted the cloture rule to end debate. The study finds that Judiciary chairs originally sent blue slips to home state senators to determine whether they planned to filibuster nominees.

Why would chairs want that information? Because up until 1917 there was no formal cloture rule senators could invoke to end debate. Thus, the chair of the Judiciary Committee wanted to know whether a home state senator would grind the Senate's business to a halt by filibustering the nominee. The blue slip was the primary tool to sniff out that information. According to the study, then, today's blue slip is a classic case of unintended consequences.

Different Senate Judiciary Committee chairs have also treated the blue slip differently. For much of history, Judiciary Committee chairs did not treat blue slips as dispositive. Early chairs treated them as advisory. When Sen. James Eastland, D-Miss., took over as chair in 1956, however, he allowed even a single negative blue slip to kill a nomination (likely because of racial politics). After Eastland's departure, things changed. Between 1979 and 1981, Sen. Ted Kennedy, D-Mass. chaired the committee and altered blue slip policy to clear the way for more women and minorities to sit on the federal bench.

Oftentimes, judiciary chairs treated blue slips differently depending on whether they shared partisan affiliation with the president. Democrats, for example, complained that Sen. Orrin Hatch, R-Utah, treated blue slips differently under President Clinton than he did under President Bush. The blue slip has anything but a stable history.

Taken together, the data and history show the blue slip is riddled with ideological gamesmanship conducted in relative secrecy. Whether it should be removed or simply reformed is a question for others. Clearly, there are strong normative reasons to keep the blue slip. But equally clear is that the modern blue slip has become a cartoonish version of its better self: a tool that angry Hulks use to smash their opponents.

Ryan Owens is a political science professor at the University of Wisconsin-Madison.

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