If Mr. Mueller promptly exonerates the President, the presence of partisan Democrats on the investigative team will certainly lend credibility to Mr. Mueller’s findings. In a similar way, the presence of Trump donors on the team would bolster Mr. Mueller’s case if he ended up claiming that the President did something wrong.

Conscientious people—and even attorneys—can put their politics aside and perform honest service. But rather than deciding which political partisans to engage in this work, shouldn’t Mr. Mueller be seeking people who don’t have a particular rooting interest in the outcome? For both the appearance and the reality of a fair inquiry, an independent staff would seem to be an obvious objective when assembling the team.