President Barack Obama meeting with Congressional leaders in the Old Family Dining Room of the White House in Washington. From left are, House Speaker John Boehner of Ohio, Obama, Senate Majority Leader Harry Reid of Nev., and Senate Minority Leader Mitch McConnell of Ky. (Evan Vucci/AP)

PRESIDENT OBAMA on Saturday nominated Loretta Lynch, the U.S. attorney in Brooklyn, to be the next attorney general. In so doing he set up an early test of whether Republicans are serious about governing in a spirit of cooperation.

Few issues did more to divide the Senate over the past several years than the vetting of Mr. Obama’s judicial and executive-branch nominees. Republicans held up several high-profile presidential picks on what struck us as frivolous grounds, and their tactics slowed consideration of lower-profile cases, too. Democrats accused them of bringing Senate confirmation nastiness to a new low. Republicans defended themselves by claiming that the Democrats had been as bad or worse in opposing President George W. Bush’s nominees. Even if true, that was hardly a defense.

During a fight over staffing the appeals courts, Senate Majority Leader Harry M. Reid (D-Nev.) took the audacious (we would say reckless) step of forcing a Senate rules change, reducing the bar for confirmation to a simple majority vote in the chamber for all but Supreme Court justices. That move upped the partisanship and didn’t fix the confirmation system. Would-be public servants are still languishing.

Nowhere is this more evident than in the number of U.S. embassies operating without ambassadors because the Senate process has been so gummed up: Thirty-seven ambassadorial nominees have had hearings before the Foreign Relations Committee yet remain unconfirmed, according to the American Foreign Service Association. Over the summer, when the number of ambassadorial vacancies was even higher, Republicans blocked an attempt to confirm a block of picks en masse.

Then there is Vivek Murthy, Mr. Obama’s surgeon general nominee, who has encountered GOP resistance because he dared point out that gun deaths are a public health problem.

Sen. Mitch McConnell (R-Ky.), the presumptive majority leader in the next Congress, has said he wants to return the Senate to “regular order” and that Ms. Lynch’s confirmation process will be “fair.” Among other things, that should mean embracing the notion that the Senate should not attempt to micromanage or slow-walk the process of staffing up the government, particularly the executive branch, where the president deserves deference in assembling his team. Senators should ensure that presidential nominees are qualified and competent, not that they will run agencies as Republican appointees would.

Ms. Lynch was a careful choice with more than ample qualifications and no obvious political baggage. Republicans are not attacking her, at least so far, but they are pushing to hold off her confirmation proceedings until the new, GOP-run Senate convenes in January, while harping on a long list of complaints about Mr. Obama and current Attorney General Eric Holder — on immigration enforcement, the Defense of Marriage Act and other matters — in seeming preparation to demand answers from Ms. Lynch.

It shouldn’t take months to assess Ms. Lynch’s competence. Even more important, her hearings should not turn into a trial of Mr. Holder or the president. That would be insulting to a woman who has her own record on which she can and should be judged. Republicans should cooperate with Democrats to process as many pending nominations as possible before this Congress ends and then behave responsibly when they take over.