It would have been hard for President Obama to nominate a less controversial person to join the U.S. Court of Appeals for the District of Columbia Circuit, the second-most-important court in the land. So why are a lot of Republicans probably going to vote against moving forward with Patricia Millett’s nomination on Thursday?

Ms. Millett is one of three people the president picked to fill three open slots on the court, a high-profile perch in the judiciary that reviews weighty matters such as regulation of Wall Street and the environment. A lawyer who has extensive experience arguing cases before the Supreme Court, she has gold-plated bipartisan credentials, having served in the Justice Department under the presidencies of Bill Clinton and George W. Bush. A raft of legal luminaries has endorsed her, including conservative former solicitors general Ted Olson, Paul Clement and Kenneth Starr . Even conservative GOP senators admit she is well-qualified.

But instead of being judged on her merits, Ms. Millett may well end up a victim of a GOP campaign against allowing any more of Mr. Obama’s nominees onto the D.C. Circuit. Though Republicans pushed to fill its 11 seats when George W. Bush was president, they now argue that it doesn’t need more than its current eight judges, and that Mr. Obama is trying to “pack” the court. Some have backed a bill from Sen. Charles E. Grassley (R-Iowa) that would strip the court of its vacancies rather than consider the president’s duly appointed picks to fill them.

A war of dueling numbers on the D.C. Circuit’s workload has ensued. Republicans insist that it doesn’t take as many cases as other appeals courts do. Democrats respond that the D.C. Circuit must consider more complex cases than others.

But the answer doesn’t matter. Even if Republicans are right, they shouldn’t insist on altering the size of a court only when it’s a Democratic president’s turn to pick judges or filibuster highly qualified nominees on that pretext. These moves are transparently self-serving, and would encourage similar behavior by Democrats against Republican presidents. The recent history of the confirmation process is a steady descent into unreasonable partisanship; if acted upon, the Republicans’ position would be another step down. It might also provoke another unnecessary battle over Senate rules, which could reshape the chamber in ways both parties would regret.

If Republicans are genuinely concerned that the D.C. Circuit has too many slots allotted to it, the fair way to trim it down would be to limit future presidents from filling seats that come open in the next presidential term and thereafter. In the meantime, President Obama’s picks deserve the same fair treatment and respect as any others. Under that standard, Ms. Millett should fly through confirmation on Thursday.