THIS ONE shouldn’t be complicated. The fourth year of President Obama’s four-year term has just begun. Senators are elected to six-year terms, and all of them have at least 11 months still to serve. The death of Justice Antonin Scalia has created a vacancy on the nine-member Supreme Court. The Constitution tells the president to nominate justices and senators to confirm or reject those nominees.

Yet within hours of Justice Scalia’s passing, Senate Majority Leader Mitch McConnell (R-Ky.) had declared that the Senate will not do its job. Partisans on both sides began scouring the historical record for precedents concerning judicial nominations in the fourth year of presidential terms. That record, though scant, does not bolster Mr. McConnell’s case, in our view, and it is almost beside the point. The issue is whether the Senate will continue its downhill slide toward politicization of judicial nominations or seize an opportunity to regain some respect as a deliberative, constitutional body.

Chief Justice John G. Roberts Jr. noted in a public appearance in Boston recently that Justices Scalia and Ruth Bader Ginsburg were both confirmed by overwhelming bipartisan majorities, while more recent nominees — Samuel A. Alito Jr., Sonia Sotomayor and Elena Kagan — were approved on largely party-line votes, though they were no less qualified and no more ideological than their predecessors. “That suggests to me that the process is being used for something other than ensuring the qualifications of the nominees,” the chief justice said. He said the politicization in turn has a negative effect on how Americans view the court, The Post’s Robert Barnes reported.

We have rebuked both parties for their part in the deterioration, including Democrats for the poisonous way they engineered changes in confirmation rules for lower-court judges, and both sides for blocking qualified nominees. Each bit of bad behavior is taken as a justification to do something worse when the other side gets a chance. Now Mr. McConnell’s blanket refusal to consider a nominee has the potential to start a new downward cycle. We understand that, if positions were reversed, Democrats who are now piously invoking the Constitution would be articulating a different view. But that doesn’t make Mr. McConnell right. If a Republican is elected president in November and Democrats recapture the Senate, what is to prevent them from refusing to vote at all? Nothing in the Constitution decrees when a lame duck becomes lame. Meanwhile, the nation is harmed by having a hobbled court.

Mr. McConnell’s ill-considered fiat has put a spotlight on a handful of Republican senators facing tough reelection battles in purple states who, it is conjectured, may be politically harmed if they refuse to treat a court nominee with respect. We would shine the spotlight instead on freshman Republican senators, who bear no scars from past judicial fights and who in some cases ran for office on a platform of renewed fealty to the text of the Constitution. Mr. Obama should nominate the best qualified person he can find, not one chosen for maximum political advantage. Then senators should insist that they be given the opportunity to do what their states elected them to do: evaluate the nominee fairly, and vote aye or nay.