Germany's main opposition party piled pressure on Chancellor Angela Merkel on Sunday over a scandal dogging her handpicked candidate for president, calling for new elections if he were to step down.

As new revelations emerge almost daily about Christian Wulff, 52, and a home loan affair that has dominated the country's media for weeks, a senior member of the opposition Social Democrats (SPD) sought to shift the heat onto Merkel.

The party's secretary general, Andrea Nahles, told the Bild am Sonntag weekly: "If Wulff resigns, then there should be new elections ... the Wulff affair is also a Merkel affair."

"Christian Wulff is not up to the office of federal president. Staying in office, no matter what happens? That behaviour is not acceptable ... I have serious doubts that he will survive this affair," added Nahles.

The SPD's parliamentary group leader, former foreign minister Frank-Walter Steinmeier, also chimed in, noting Merkel had pushed through Wulff's election to the largely ceremonial post in 2010.

"She cannot just act as if she has nothing to do with this whole affair and as if the federal president is in a different political sphere," Steinmeier told Sunday's edition of the Tagesspiegel.

The scandal broke in December when mass circulation Bild reported that Wulff failed to declare a loan he had obtained from the wife of a wealthy tycoon when he was the leader of the state of Lower Saxony.

On learning the story was about to break, Wulff left furious voicemail messages with the Bild's chief editor and reportedly also the chief executive of the paper's publisher.

According to Spiegel newsweekly, Wulff asked to discuss the story on his return from an official trip "and then we can decide how we see things and then we can decide how we should wage war."

As recently as Friday, Merkel's spokesman said she had "great esteem" for Wulff both as a person and as a president, but the scandal is an increasingly unwelcome distraction as she seeks to focus on the eurozone debt crisis.

And top government officials scrambled to deny reports over the weekend that talks had taken place over a possible successor to Wulff.

However, as hundreds protested outside the official presidential residence in Berlin over the weekend and polls on Sunday showing Germans believed he had lost credibility, Wulff himself reportedly sought to shrug off the scandal.

"In a year, this will all be forgotten," he told staff members, according to the Bild am Sonntag.