Germany’s 37-year-old economy minister, Karl-Theodor zu Guttenberg, could become a liability for Chancellor Angela Merkel in September’s election thanks to his open criticism of the government’s 11th-hour rescue of carmaker Opel.

Guttenberg, a rising star in Merkel’s conservative camp, had argued for an Opel insolvency in the days preceding the deal.

He astonished reporters when he expressed objections to the agreement just minutes after the announcement in the early hours of Saturday that German taxpayers would help tide over Opel’s operations until General Motors concluded an agreement to sell Opel to a group led by Canadian supplier Magna.

“I want to say that, in a very difficult discussion process … I personally came to a different view of the risks,” said Guttenberg, a Bavarian who has been economy minister for less than four months. There are no Opel plants in Bavaria.

There are strong rumours he threatened to resign after his opposition to the deal was ignored and at the weekend Guttenberg, seen by some as a possible future chancellor, continued his attack.

“The threat is the state can be blackmailed if it is overly generous with help even once,” the media-friendly minister told the Welt am Sonntag newspaper in remarks that have given ammunition to his political foes.

Guttenberg’s comments have gone down well with some in his own conservative camp and could even strengthen his position with them in the long run, especially if he is proved right and the deal turns out badly. 

Merkel has backed him so far, saying on Monday she was grateful to him “for repeatedly sticking his finger in the wound”.

However, he has made himself an easy target for the Social Democrats (SPD), who share power with Merkel’s conservatives but will oppose them in September’s election.

Ministers have been lining up to attack him. SPD Chairman Franz Muentefering accused him of undermining Opel’s negotiating position. Others have portrayed him as a ranting right winger.

Probably most damaging of all, former SPD Chancellor Gerhard Schroeder, one of Germany’s most savvy campaigners, has ridiculed him as the “Baron from Bavaria”, a name likely to stick.

He has homed in on two things that anger many Germans — his aristocratic roots and his wealth, which could prove to be a drawback during an election campaign fought against the backdrop of a deep recession.

Probably more important, Schroeder has reminded voters that Guttenberg is Bavarian. No member of Bavaria’s conservative Christian Social Union (CSU) , sister party of Merkel’s Christian Democrats, has become chancellor, largely because of a strong antipathy among many Germans towards the prosperous, proud southern German state.