IF YOU pass the vast, glass-walled chancellery in Berlin at night, you will often see a solitary room illuminated high on its northern side. That is Peter Altmaier's office, a few strides away from Angela Merkel's own eyrie on the other side of the seventh floor. An intellectually brilliant, multilingual, workaholic bastion of the CDU's liberal wing, many consider the chancellery director the most powerful man in the city.

All the more so since yesterday, when Mrs Merkel added a new job to Mr Altmaier's portfolio: he will now write her manifesto ahead of the election in September. The news triggered volcanic reactions from parts of political Berlin. To understand him, his role and his political significance is to understand how Germany is run in 2017.

On the surface, the drama at his latest elevation is about practicalities. Wolfgang Kubicki, vice-chair of the liberal FDP, huffed that Mr Altmaier could not be both chancellery director and manifesto writer; this would be "blatantly unconstitutional" and possibly grounds for legal action, he warned. "He can't simultaneously be an honest broker of this coalition government and write the CDU manifesto", added Carsten Schneider, a leading SPD MP.

Both had a point: what if a crisis breaks (say, a major terror attack) and Mr Altmaier is busy taking soundings on the CDU's new health policy? The directorship of the chancellery is a huge job in itself, much larger than chief-of-staff roles in other countries: the director co-ordinates departments and brokers compromises between them, federal states and coalition parties. He even oversees the BND, Germany's foreign intelligence agency.

The man himself hit back today, vaunting his endless capacity for work: "For three years I have been reachable and available to act at any time. That includes during emergencies, at night, at Christmas and at the weekend. That will remain the case." He has a point. Sigmar Gabriel, now Germany's foreign minister, was until recently head of a giant economy ministry, chair of the centre-left SPD and vice chancellor. There is nothing new about this cumul des mandats in Berlin.

Which begs the question, why the exaggerated horror at the news? There are three main factors.

The first is Mr Altmaier's proximity to Mrs Merkel. His stratospheric rise has happened fast: little more than a decade ago, the generously built MP from the Saarland was joking that he had no prospects under a woman with a "weakness for thin men". Then the chancellor plucked him out of obscurity.

He has since appeared in every great drama of her tenure. He was the chief whip who pushed the Greek bailouts through the Bundestag. He was the environment minister who executed Mrs Merkel's "energy revolution”; the forests of wind turbines now rising across Germany are his doing. He was the architect of her refugee gambit, flying to Ankara without the say-so of the interior ministry to thrash out deals with Turkish leaders. Now he is at the heart of Berlin's response to Brexit; earlier this month he was the only senior German minister to attend the annual Anglo-German “Königswinter” conference.

He is synonymous with Merkelism: an hero to her fans, a scourge to her enemies. In the words of Robin Alexander, the author of a new book on the refugee crisis, "those who don't dare attack Merkel directly take aim at Altmaier".

The second factor is his tense relationship with CDU traditionalists. As a new MP in the 1990s Mr Altmaier made enemies in his party by advocating the rehabilitation of armed-forces deserters and new measures against rape within marriage. He was integral to the Pizza Connection, a gang of moderate CDU and Green politicians who met at Sassella, an Italian restaurant in Bonn. On the CDU right and in its more conservative Bavarian sister party, the CSU, some suspect this betrays his real allegiances: "The problem with Altmaier is that he has spent his whole life in the wrong party...he has always thought of himself as a Green", writes one Der Spiegel journalist.

Today he is a, perhaps the, major force in a giant reshaping of German politics: the alignment of the two parties. With Mr Altmaier at her side, Mrs Merkel has overseen two left-libertarian gambits; one environmentalist and one humanitarian. Meanwhile the "realo" (moderate) wing of the Greens is now dominant: the party's two lead candidates and its one Land minister-president (Winfried Kretschmann in wealthy Baden-Württemberg) are all relatively CDU-friendly. If, as many suspect, this CDU-Green alliance advances in the coming months and years, you can be sure it will bear Mr Altmaier's fingerprints.

The third factor is that his internal opponents fear him. He works with superhuman loyalty to Mrs Merkel—and this appears to be reciprocated. Witty and spontaneous, he is the second most-followed tweeter in German politics. Some feel intimidated by his intellect: when Ahmet Davutoğlu, the former Turkish prime minister, spoke of a new German-Turkish alliance Mr Altmaier immediately ruminated on the German generals, Colmar von der Goltz and Liman von Sanders, who had helped the Ottoman sultan modernise his army in the early 20th century. He once told me he owns 600 books about Bismarck. In a party in which an uncanny number of big beasts have fallen during the Merkel chancellorship, Mr Altmaier looms especially large. Peter Tauber, the CDU general secretary widely deemed to have been demoted by Mr Altmaier's new promotion, never daunted people like his new colleague in the Konrad-Adenauer-Haus (the CDU headquarters).

Now for a disclaimer: insofar as he is a motor of a more decisive, European, liberal Germany, I tend to think Mr Altmaier is a good thing. In character he is what British politicos call a "cavalier". But whether or not you like him, he matters. As the CDU manifesto-writer, he must answer the question: what is new about Angela Merkel, after 12 years as chancellor? Moreover, while she forged a cease-fire with the CSU over the refugee crisis, Mr Altmaier must now negotiate the peace process; the joint manifesto must balance the Bavarians' demand for an upper limit on arrivals with the chancellor’s opposition to it. He will be a central force in talks after the election on September 24th.

While a new “grand coalition” of CDU and SPD seems probable, Mr Altmaier's links to the Greens would suddenly flower if the numbers for a CDU-Green-FDP government materialised (in any case, the Greens are in nine of Germany's 16 state governments.) Winning a fourth term as chancellor would put Mrs Merkel in the danger zone: her two predecessors to achieve this—Konrad Adenauer and Helmut Kohl—collapsed soon afterwards. Whether she gets this chance, and how she handles it, has so much to do with Mr Altmaier. Take him seriously.