For a French president to admit he was wrong is almost blasphemy, but it happened this week and politics here may never be the same.

Furthermore, President Francois Mitterrand's mea culpa was not merely on peripheral matters: he admitted he was wrong on economics, on the franc -- even on President Reagan.

True, Mitterrand's confessional was done with the discretion appropriate to his office; a conversation with trusted journalists that apparently was meant to be off the record was published in a small-circulation, left-wing Catholid weekly, Temoignage Chretien.

"I made the mistake of not devaluing in 1981," Mitterrand said, referring to the year in which he was elected. "I was carried away by victory. . . . Let's face it, we all dreamed a hit in 1981."

Mitterand added, "We underestimated the depth of the international recession, and I overestimated the good will of the Americans. Now I expect nothing from Reagan."

The first penitence -- over devaluation -- is not really damaging because it is blamed on advisers; the president said he knew all along he should have devalued at once.

"Only [Foreign Trade Minister Michel] Jobert encouraged me to devalue. In my heart of hearts, I felt he was right but [Prime Minister Pierre] Mauroy and [Finance Minister Jacques] Delors talked me out of it and [Planning Minister Michel] Rocard said nothing."

In the article, Mitterand blamed his hesitancy in pursuing economic austerity on the West Germans. "Right from the spring of 1982 I wanted rigor," he is quoted as saying. "But the Germans were not ready, and everyone -- economists, journalists and advisers -- were saying growth was coming back."

Perhaps the only really concrete error Mitterand recognized as his own was in trusting Reagan to help the socialist economy of France. The failure of the Versailles summit to influence U.S. interest rates, which are perceived here as being way too high, and the humiliation at Williamsburg of having to sign a Reaganesque nuclear missiles statement without getting anything in return have been Mitterand's bitterest experiences as president.

Why the humble and unpresidential self-criticism? Some observers here speculate that the probable reason is that Mitterand has been consulting the public relations expert formerly used by his successful Socialist rival in the popularity stakes, Rocard. The matter is urgent because in the latest poll 51 percent said they were dissatisfied with the president's performance, and only 35 were satisfied.

The expert message being beamed to the electorate is that Mitterrand may have been wrong, but it was the fault of ministers, and, contrary to what many here long have believed, Mitterand does, "in his heart of hearts," understand economics.