Ten months into the FBI's investigation of the Trump-Russia affair, and four months into a bipartisan probe by the Senate Intelligence Committee, Deputy Attorney General Rod Rosenstein has decided to appoint a special counsel, former FBI Director Robert Mueller, to investigate the case. Here are several reasons why it is a bad move:

1) It roils and extends an investigation that, on Capitol Hill at least, was moving toward a conclusion on the allegation at the heart of the matter: whether Donald Trump or his associates colluded with Russians to fix the 2016 election. By the time Rosenstein made his move, Senate investigators were becoming more and more confident that there was no collusion. They couldn't say for sure — they were cautious to say something could still come up — but the collusion charge was looking like a non-starter. There were also indications that the FBI's counter-intelligence investigation was finding the same thing. Now, the appointment of Mueller essentially pushes the re-start button on the investigation.

2) It raises the possibility of the Trump-Russia investigation needlessly lasting the entire duration of the Trump presidency. The last time there was an even slightly similar situation, the appointment of special counsel Patrick Fitzgerald to investigate the Valerie Plame-CIA leak case, the mostly fruitless probe went on for years and years. (One of the loudest voices demanding the appointment of a special counsel back then, as now: Sen. Charles Schumer.) Prosecutor Fitzgerald actually knew the identity of the leaker when he was appointed on December 30, 2003, yet kept the investigation going until March 2007 with the conviction of Scooter Libby, not for leaking but for perjury and obstruction of justice. There were no charges on the core event of the case, the Plame leak. If the Mueller investigation lasts as long as the CIA leak case, it will extend well into 2020.

3) It risks going down rabbit holes. The Rosenstein order appointing Mueller is ambiguous about the scope of the investigation. The order authorizes Mueller to investigate "any links and or coordination between the Russian government and individuals associated with the campaign of President Donald Trump," and also "any matters that arose or may arise directly from the investigation." As one Republican senator pointed out in a private conversation Wednesday night, a lot depends on the meaning of "matters" and "directly." If Mueller interprets those words expansively, look for the investigation to go in unpredictable directions, and certainly continue current probes into Michael Flynn's work for Turkey and Paul Manafort's business arrangements in Ukraine from years ago.

4) It is ill-suited to handle the Trump-Russia affair. Many informed observers have noted that it's unlikely there is an actual crime at the heart of the case. To the degree that it is an investigation of Russian efforts to influence the election, it's a national security case. To the degree that those efforts might, or might not, have influenced the 2016 voting, it's a political matter. Congressional committees, or even an independent commission, are the better choices for that kind of investigating.

5) It is going to mess with the Senate Intelligence Committee investigation. Committee members were proud of the bipartisan cooperation they had shown in the investigation up to now. They could see the end in the distance. Now, the Senate will be working under the heavy burden of the Mueller investigation. Any witnesses who weren't already lawyered up will get lawyered up. Any witnesses who haven't already talked to the Senate will be unlikely to do so as they strive to do nothing that might cause them trouble with Mueller. In short, the Senate investigation will slow down and become more difficult.

6) It will make it harder for the public to learn what happened. If Mueller charges anyone, the indictment and trial will of course reveal what happened in that corner of the case. But if Mueller makes no charges on the key issue in the case — collusion — he won't issue a report telling the public what happened. He just won't bring any charges.

7) It is likely to end in murkiness and irrelevance. If the investigation ends with prosecutions, they will likely be for actions not related to the key Russia-Trump question — Flynn and Turkey and Manafort and Ukraine — or in process crimes like making false statements. There will be no explosive, dramatic ending, no great satisfaction for Trump enemies and no vindication for Trump supporters. Just a long, long, unnecessary slog.