The Washington Post's Devlin Barrett explains the Justice Department's decision to appoint Robert Mueller as special counsel to investigate possible connections between the Trump campaign and Russian officials. (Peter Stevenson,Jason Aldag,Whitney Leaming/The Washington Post)

The Post reports:

The Justice Department has decided to appoint a special counsel to investigate possible coordination between Trump associates and Russian officials seeking to meddle in last year’s election, according to Deputy Attorney General Rod Rosenstein.

Robert Mueller, a former prosecutor who served as the FBI director from 2001 to 2013, has agreed to serve in the role, Rosenstein said. The move marks a concession by the Trump administration to Democratic demands for the investigation to be run independently of the Justice Department. Calls for a special counsel have increased since Trump fired FBI Director James B. Comey last week.

This move became inevitable once Rosenstein was embroiled in the Comey firing. Indeed, the appointment of a special counsel may be seen as recognition that those events — in which Rosenstein is both a witness and a participant — must be examined. Rosenstein’s letter is certainly broad enough to cover possible obstruction-of-justice charges against the president, attorney general and other White House aides. (Mueller will look at “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump’’ as well as “any matters that arose or may arise directly from the investigation’’ and, as The Post reported, “any other matters that fall under the scope of the Justice Department regulation covering special counsel appointments.'”)

Rosenstein could not have picked a better choice. As a former FBI chief, Mueller knows precisely what to look for and how to conduct an exhaustive investigation. Rosenstein therefore will be lauded for stepping aside and deemed to have recovered his reputation, sullied by involvement in Comey’s firing.

President Trump tweeted his objection to the appointment of a special counsel to investigate possible collusion between his associates and the Russian government during the 2016 presidential election. (Victoria Walker,Jayne Orenstein,Dalton Bennett,Jenny Starrs/The Washington Post)

In one sense, this is a tremendous boost for Democrats who have been imploring Rosenstein to appoint a special prosecutor. They rightly argue that the Justice Department itself is now implicated in potential wrongdoing, as is the president of the United States. However, Republicans in Congress also should breathe a sigh of relief. They will still be hounded to appoint a select committee to oversee the entire matter, if not an independent commission, but they no longer have to resist the demands for a special prosecutor.

Frankly, the White House has every reason to panic. No one will intimidate or throw Mueller off course. The seriousness of the probe could not be more clear. A pall will soon fall over the White House as every member of the staff, the vice president and the president will brace themselves for interrogation, production of potentially damaging documents and, incidentally, big legal bills.

Coming just as the president prepares to leave on a foreign trip, appointment of a special prosecutor comes as one more huge blow to his standing and ego. He now goes overseas — something the homebody Trump reportedly dreaded — as a wounded president with an uncertain future. Nothing could be more disconcerting to allies than dealing with an impulsive, ignorant president — one whose future is far from certain. Trump can whine about the unfair press, as he did at the Coast Guard commencement Wednesday, but he has no one but himself to blame for his predicament.

The 100-day mark was the end of the beginning of Trump’s term. The appointment of a special prosecutor just four months into his presidency might be seen as the beginning of the end.