How the Senate should treat one of its own is always complicated. While they may often disagree on fundamental matters, senators spend time together on the floor, in the gym and at countless hearings, and they often develop amiable and respectful relationships over the many years they coexist. Being a member of the club is usually thought to entitle a senator to a degree of deference when it comes to confirmation for a high-level administration post or an appearance before the Senate in one capacity or another. Consider that Hillary Clinton was confirmed for secretary of state in 2009 on a vote of 94-2.

But having served in the Senate is not always a guarantee. The most famous example was John Tower, the conservative Texas senator who was rejected as President George Bush’s nominee for defense secretary in 1989 after former colleagues deemed him unfit for the job because of character issues and potential business conflicts. Former senators have had a tough go in more recent years as well. Chuck Hagel, a former Republican senator from Nebraska, took a beating from his former side of the aisle when he was nominated as defense secretary by Barack Obama in 2013, though he survived to win the job.

When Mr. Sessions, an early supporter of Donald J. Trump, was picked to lead the Justice Department, Democrats knew they had little chance to block him, given Senate changes that meant filibusters on nominations could be broken with a simple majority vote. They raised his civil rights record — the Senate rejected his nomination for a federal judgeship in 1986, citing racially charged comments and actions — but focused more on his willingness to challenge Mr. Trump should he exceed his authority. Mr. Sessions promised not to be “a mere rubber stamp” for the president.

“If an attorney general is asked to do something that’s plainly unlawful,” he said, “that person would have to resign ultimately before agreeing to execute a policy that the attorney general believes would be unlawful or unconstitutional.”

It was not obvious at the time, but the question that has drawn Mr. Sessions into trouble was a bit of an offhand one from Senator Al Franken, Democrat of Minnesota. He pressed Mr. Sessions on new reports of possible communication between the Trump campaign and Russians and asked what he would do as attorney general if those reports were confirmed.