HILLARY CLINTON’S election as president is hardly a sure thing. Nevertheless, progressives in the Democratic Party, feeling emboldened by the strong showing of Sen. Bernie Sanders (I-Vt.) in the primaries, are preparing their demands for a second Clinton administration. Following the old Washington adage that “personnel is policy,” unions, activists and think-tankers, under the informal auspices of Sen. Elizabeth Warren (D-Mass.) and Mr. Sanders, are putting together lists of acceptable candidates for senior administration jobs — and a blacklist of unacceptable ones.

A certain amount of factional horse-trading over nominations is par for the course in politics; it’s especially so in a year when Ms. Clinton is reliant on support of the Warren-Sanders troops. We might add, though, that if Ms. Warren wanted to ensure her choices got picked, she could have run for president herself, as many urged her to do; Mr. Sanders did run, but failed to get more votes than Ms. Clinton, so his entitlement to determine administration staffing is a bit debatable as well.

The real problem, though, is not the fact of the lobbying but its nature: Ms. Clinton is being told, apparently, to apply a series of litmus tests to her potential nominees that include not only adherence to the Democratic Party platform, the drafting of which she largely (though not totally) ceded to Mr. Sanders, but also avoidance of any connection to the financial industry — notwithstanding whatever expertise it might confer. These conditions, or others unknown, may disqualify two able potential treasury secretaries, Sheryl Sandberg of Facebook and current Federal Reserve Board member Lael Brainard. Ms. Warren also wants Ms. Clinton to grant a demand that President Obama has so far refused: to fire Securities and Exchange Commission chair Mary Jo White for her refusal to advance a regulation — largely symbolic and arguably irrelevant to the SEC’s statutory mandate — requiring publicly traded companies to disclose their political donations.

That would set a troubling precedent by linking a current official’s tenure at an independent regulatory agency to his or her policy decisions. The advance veto on future nominees that Ms. Warren and Mr. Sanders are seeking, if carried to its logical extreme, would make them de facto usurpers of presidential power. The left’s model is its successful campaign against Mr. Obama’s choice of former investment banker Antonio Weiss as undersecretary of treasury for domestic finance in 2014. Under fire from Ms. Warren because of his role in the now-forgotten merger of Burger King and a Canadian company, Mr. Weiss withdrew early last year without a Senate hearing. In the end, this blackballing accomplished nothing practical: Mr. Weiss accepted a Treasury job that did not require Senate confirmation, and ably shepherded bipartisan legislation to restructure Puerto Rico’s finances. Meanwhile, the job for which he was originally tapped remains empty. Such end runs could take place in a Clinton administration, too, thwarting the left but at the risk of muddied lines of accountability in the bureaucracy.

People should be nominated based on their individual merits by the person with clear constitutional power to do the nominating. If elected, Ms. Clinton should declare from the outset that is how she will operate.