Given that context, one might expect Mr. Trump to be clamoring for details that would eliminate any suspicion that his administration is in league with an enemy. Instead he has waged an unhinged attack on the intelligence agencies themselves, praising President Vladimir Putin of Russia at every turn and pointing fingers everywhere but at himself, while refusing to take a single step to resolve questions about his administration’s ties to Russia.

Hence the urgent need for high-level congressional intervention. The ideal vehicle would be an investigative committee of senior senators from both parties as well as members of the House. Some Senate Republicans are beginning to step up. Senator Richard Burr, chairman of the Senate Intelligence Committee, has already said his committee will investigate the election hacking. Chuck Grassley, chairman of the Senate Judiciary Committee, and Dianne Feinstein, the panel’s top-ranking Democrat, are asking for a briefing and transcripts of Mr. Flynn’s calls to the Russian ambassador.

Senators John McCain, Lindsey Graham and Pat Roberts favor a broader investigation. John Cornyn, the Senate majority whip, has also raised the possibility of an investigation by Senate committees with jurisdiction over the intelligence community.

The Democrats would obviously be on board — Chuck Schumer, the Senate Democratic leader, has also called for the Senate Intelligence Committee to lead a bipartisan inquiry. The person who needs to make this happen is Mitch McConnell, the Senate majority leader. Whatever form the committee takes, as Mr. Schumer said on Wednesday, all members must be granted equal access to “intelligence officials, transcripts and documents that they need to answer critical questions, and they must be permitted to make their findings public to the maximum extent possible.”

Admittedly, this is hoping for a lot from a Republican leadership whose natural inclination is to protect the president. This week, for instance, congressional Republicans closed off one avenue to forcing the release of Mr. Trump’s tax returns, which he has refused to divulge and which could help prove to Americans that he is not indebted to Russian financial entities. (It bears repeating, in this regard, that Mr. Trump didn’t fire Mr. Flynn this week for chummily discussing American sanctions on Russia with Moscow’s ambassador, or for lying about it. Mr. Trump knew all that for weeks. He fired Mr. Flynn after both of them got caught.)