Wulff is facing a chorus of demands that he explain why he tried to suppress a newspaper story about loans from wealthy friends he used to finance his private home.
The three-week-old controversy over the loans flared anew this week with news that Wulff had phoned Germany's biggest newspaper, Bild, on 12 December. He left a voicemail on the mobile phone of chief editor Kai Diekmann, threatening a "war" if the newspaper carried the story.
Bild, a tabloid selling 3 million copies daily, published the report.
The news of the phone call was reported by the rival Süddeutsche Zeitung and Frankfurter Allgemeine Sonntagszeitung. Bild explained on 2 January that it had not carried the news of the threatening message after Wulff called Diekmann to apologise.
As Wulff remained silent, opposition representatives questioned his fitness for office but avoided directly calling for his resignation.
The controversy adds to the political headache for Chancellor Angela Merkel as she scrambles to find a solution out of the eurozone crisis. Most senior figures in Merkel's party, the Christian Democratic Union, refused to speak on the record about the allegations, but said Wulff was a tough professional politician and predicted he would survive the crisis, DPA reported.
Wulff is also a Christian Democrat with close ties to Merkel.
The Free Democrats (FDP), the junior partners in Merkel's coalition, say it is up to the president to set the record straight. "I am confident he will succeed," FDP Secretary-General Patrick Döring said on Tuesday as quoted by Deutsche Welle.
Wulff's election in June 2010 was already a humiliation for Merkel. The post of president is largely ceremonial in Germany and the head of state is elected by parliament. However, it took three rounds of voting to elect Wulff, and in the first round 44 members of Merkel's party voted against him in a major blow to her leadership.
Wulff's predecessor, Horst Köhler, unexpectedly resigned on 31 May 2010 following a 'misplaced comment' he made about German military action abroad.
This is not the first time that Wulff tried to suppress a media report in his presidential capacity, German media reported. In June 2011, Wulff is said to have tried to make the Welt am Sonntag newspaper drop a story about his half-sister.
The Financial Times Deutschland said Wulff is not up to the job of German president and called for him to resign.
"His credibility has been hit by this plethora of mistakes," the influencial daily wrote.