THE CONCERNS voiced about Sen. Jeff Sessions’s (R-Ala.) nomination for attorney general cannot be written off as political grandstanding. There are substantive objections to Mr. Sessions, for years an ideological outlier in the Senate, ascending to the pinnacle of the U.S. justice system. Though the senator made a series of encouraging commitments during his confirmation hearings last week, he did not allay every reasonable worry.

Mr. Sessions was at his most impressive when he repeatedly insisted he would respect the rule of law, affirming that the attorney general must be willing to say “no” to the president and must resign if ordered to do something illegal. Mark those words. Mr. Sessions also committed to enforcing Supreme Court rulings upholding abortion rights and same-sex marriage, though he disagrees with them. He promised to recuse himself from any outstanding questions relating to Hillary Clinton’s email practices or the Clinton Foundation, given his strong comments on these matters during the campaign. Though he alarmingly supported “enhanced interrogation techniques” in the past, he acknowledged that laws barring them now must be respected.

Mr. Sessions was not quite as definitive in disclaiming Mr. Trump’s proposed “Muslim ban.” He insisted that restricting all Muslims would be wrong, but he left wiggle room when he said that those who interpret their religion to include “dangerous doctrines” should be scrutinized.

Though Mr. Sessions painted himself as a civil rights champion, the picture is mixed. He acknowledged the South’s horrific legacy of racism and called the NAACP “an organization that has done tremendous good.” But he mostly evaded specific questions about discriminatory voter ID laws, which the Justice Department must play a key role in fighting, and he was unenthusiastic about “pattern or practice” investigations against errant local police forces that have resulted in valuable reforms, particularly in the past several years. He pointed out that he voted to reauthorize the Voting Rights Act, but his Democratic interlocutors reminded him that he also opposed fixing a core section after the Supreme Court gutted it.

Most concerning was Mr. Sessions’s habit of pleading ignorance to avoid taking clear positions on some pressing issues. “I have done no research into that” was his response when asked if the Russians conducted election hacking, even though he expressed confidence in intelligence agencies who have taken a clear position. Similarly, he said he did not know the details of North Carolina’s voter ID law, currently at the center of the most significant case in the country on the subject, and declined to say much about it.

Both of these examples go to critical questions about how Mr. Sessions will conduct himself. Will he live up to his commitment to stand up to the White House when the president or his staff is wrong? Will he treat civil rights enforcement as the core Justice Department responsibility it is? As attorney general, Mr. Sessions will have the opportunity to prove he is as serious about these principles as he claims.