Sherrilyn Ifill is the president and director-counsel of the NAACP Legal Defense & Educational Fund Inc. The views expressed in this commentary are hers. View more opinions on CNN.

(CNN) The ongoing investigation into President Donald Trump's conduct in the 2016 election is not just an inquiry into whether he or his associates committed crimes. It is a test of America's commitment to the basic principle that our justice system should treat every citizen equally.

If the US Senate is serious about upholding that principle, it will delay Judge Brett Kavanaugh's Supreme Court confirmation hearing until special counsel Robert Mueller has completed his investigation. If it does not, it risks tarnishing the Supreme Court and undermining the rule of law -- blows from which our nation will not easily recover.

Trump has repeatedly denounced the special counsel's investigation as a partisan "witch hunt." But the investigation's findings so far suggest otherwise. Two of Trump's former campaign aides and his first national security adviser have all pleaded guilty to federal crimes. The President's former campaign manager has been convicted of financial fraud. And federal prosecutors recently secured a guilty plea from Trump's former personal lawyer, Michael Cohen, who said he made illegal campaign finance payments at the direction of then-candidate Trump.

To be sure, we don't yet know whether the President himself committed any crimes. But until the investigation is finished -- and it must be allowed to finish -- there exists a real possibility that some related issue, such as the President's obligation to obey a subpoena, will find its way before the nation's highest court. If that happens, the American people must be able to trust that each justice will vote according to the dictates of the law and the demands of the Constitution, not out of loyalty to Trump.

The only way to ensure that trust is to refuse to consider any Supreme Court nominee put forth by this administration until the investigation is complete. Ordinary citizens don't get to appoint the judges that will hear their cases. The President should be treated no differently.

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