The Trump campaign consorted with Russian intelligence services in its efforts to influence the 2016 presidential campaign in favor of Donald Trump and then, before the new president took the oath of office, a top surrogate clandestinely worked with Russia on policy matters.

That troubling picture emerged Friday when Trump’s former national security adviser, Michael T. Flynn, pleaded guilty to lying to the FBI about conversations with the Russian ambassador.

Special Counsel Robert Mueller’s investigation has now reached into the White House. Most disconcerting for the president was Flynn’s announcement that he had reached an “agreement to cooperate with the special counsel’s office.”

It has been a dizzying few weeks. At the end of October Trump’s former campaign chairman, Paul Manafort, was arrested on allegations that he laundered millions of dollars through overseas shell companies. According to court documents, Manafort sought to hide the millions he received for working to elect pro-Russian candidate Viktor F. Yanukovych as Ukraine’s president.

Also arrested on related charges was Manafort associate Rick Gates.

Most significant, however, was the guilty plea of George Papadopoulos, a former foreign policy adviser to the Trump campaign, for lying to the FBI about his contacts with Russian intermediaries to gain dirt on Trump’s Democratic opponent, Hillary Clinton. Papadopoulos is also cooperating with Mueller.

Now Flynn has admitted to lying to the FBI about the nature of two discussions he had with Russian Ambassador Sergey I. Kislyak. They were not routine, as Flynn had alleged, but dealt with sanctions against Russia and with a pending United Nations vote, wholly inappropriate behavior given Barack Obama was still the president.

According to court documents, Flynn urged Moscow not to retaliate against sanctions imposed by Obama for Russian interference in the U.S. election, but instead await the arrival of the Trump administration. The next day Russian President Vladimir Putin announced there would be no retaliatory sanctions, a move praised by Trump in a tweet: “Great move on delay (by V. Putin).”

With the noose tightening, Congress must be prepared to reverse any attempt by the president to remove or weaken the special counsel.

This is now as serious as any past White House scandals. It is more troubling because it is the first to involve alleged collusion with a competing national power.

The Day editorial board meets with political, business and community leaders to formulate editorial viewpoints. It is composed of President and Publisher Timothy Dwyer, Executive Editor Izaskun E. Larraneta, Owen Poole, copy editor, and Lisa McGinley, retired deputy managing editor. The board operates independently from The Day newsroom.