In light of the stunning events of the past week, the question is not whether the Trump administration’s ties to the Russian government need to be investigated immediately and fully — clearly they do. It’s who will be in charge of that investigation?

The Republicans in Congress can’t decide whether they would rather act like a responsible, independent branch or just the friendly legislative arm of the White House. Jason Chaffetz, chairman of the House oversight committee, would sooner investigate a cartoon character named Sid the Science Kid than any allegations relating to President Trump.

The prize for partisan candor goes to Senator Rand Paul of Kentucky, who said on Tuesday, “We’ll never even get started with doing the things we need to do, like repealing Obamacare, if we’re spending our whole time having Republicans investigate Republicans.”

James Comey, the embattled F.B.I. director, can’t be trusted to be a neutral investigator, either — not after his one-sided interference in the 2016 election compromised the bureau’s integrity and damaged Hillary Clinton’s campaign in its final days. Anyway, Mr. Comey reports directly to the attorney general, Jeff Sessions, who was not only Mr. Trump’s first and most ardent supporter in the Senate, but the chairman of the Trump campaign’s national security advisory committee.

Despite his closeness to Mr. Trump, Mr. Sessions has said he sees no reason to recuse himself from any inquiry into the relationship between the president’s top aides and Russia. Mr. Trump’s unexplained allegiance to that country and its thug of a president, Vladimir Putin, has been a major concern from the start of his candidacy. But the scope of a potential investigation expanded sharply in the last four days, with the firing of Mr. Trump’s national security adviser, Michael Flynn, for lying to the White House about his contacts with the Russian ambassador, and the news that members of the Trump campaign’s inner circle were in repeated contact with Russian intelligence agents last year, at the same time that Russia was actively attempting to swing the election to Mr. Trump.

There is, in fact, only one person who could conduct such a high-profile, politically sensitive investigation fairly and completely — a special prosecutor.

Some Republican senators have recognized the need for an investigation, and it would be right for the Senate to move ahead in its role as a check on the executive.

But the need for an independent actor who can both investigate and prosecute criminal wrongdoing in the executive branch is clear, because the attorney general and the Justice Department cannot be reliably impartial about their own bosses. Of course, what’s simple in theory has been politically fraught in practice. In scandals from Watergate to Iran-contra to Whitewater and Monica Lewinsky, special prosecutors have butted heads with presidents and their staffs, sometimes with calamitous results.

A 1978 law, the Independent Counsel Act, created a mechanism for appointing special prosecutors who were empowered to investigate broadly and protected from presidential meddling. But the law expired in 1999 amid partisan dispute; today only the attorney general has the power to appoint a special prosecutor.

In this case, the need couldn’t be more obvious. For starters, did Mr. Trump order Mr. Flynn, directly or indirectly, to discuss sanctions with the Russian ambassador? If not, why did he not fire Mr. Flynn weeks earlier, when he apparently first learned of his lies? Were Mr. Trump’s aides colluding with Russian agents during the campaign? Perhaps most important are Mr. Trump’s tax returns, which could tell us whether he is beholden to, and thus compromised by, the Russians? House Republicans, assuming their standard supine stance toward Mr. Trump, voted on Tuesday against requesting the returns from the Internal Revenue Service; a special prosecutor would not feel so politically constrained.

It’s never easy to conduct robust, independent investigations of the most powerful people in the world, but it is one of the foundations of a functioning democracy. The concern is particularly great in the case of the Trump administration, which seems uninterested in telling the truth in matters large and small.

Mr. Sessions must appoint a special prosecutor, and he knows why. As an article published on Fox News’s website days before the election said, “Appropriate response when the subject matter is public and it arises in a highly charged political atmosphere is for the attorney general to appoint a special counsel of great public stature and indisputable independence to assure the public the matter will be handled without partisanship.”

The article, which called for an investigation into Hillary Clinton’s private email server and pay-to-play allegations surrounding the Clinton Foundation, argued that Loretta Lynch, then the attorney general, could not serve as a neutral arbiter, given her impromptu meeting with Bill Clinton on her airplane earlier in the year. One of the article’s co-authors was Jeff Sessions.