"Štefanec is not a person who should lead the Commission. We urge him to resign," the opposition New Slovenia (NSi) said Monday, adding that appointing a party member to the post was "at least tasteless if not irresponsible".

The Democrats (SDS) suggested the new leadership continued a long-standing tradition of political bias. "The commission remains a body for the sanctioning of political opponents that will receive instructions from behind the scenes," deputy Jože Tanko told the press.

Tanko suggested Pahor made a big mistake in going ahead with the appointment. "This is evidence that the act on integrity and prevention of corruption is a mess. We don't need such a commission."

The People's Party (SLS) called for Štefanec's resignation as the "only hope for Slovenia to take on the combat against corruption seriously and to build the rule of law".

SLS leader Franc Bogovič said Štefanec was in no position to head the anti-graft watchdog considering that he had until recently been member of the biggest government party, whose "frozen president" remains Zoran Janković, a person whom the watchdog exposed in a damning report in January 2013.

Even the coalition Pensioners' Party (DeSUS) has joined the appeal for resignation, with party president Karl Erjavec saying that the watchdog is "finished" if he does not step down, as nobody will believe the commission is credible and politically unbiased.

It is "unfair" to President Pahor that Štefanec kept his PS membership quiet, Erjavec said before a meeting of coalition party heads, noting that Pahor may not have picked him if he had known about it.

The Social Democrats (SD) and Citizens' List (DL) were more reserved.

SD president Igor Lukšič said Štefanec should decide himself whether the controversy is too much of an obstacle, but he was quick to point out that party membership as such cannot be a problem in advance.

Indeed, Lukšič suggested that if the scepticism regarding the anti-graft watchdog continues, it merits an analysis whether Slovenia needs such a body in the first place.

Rihard Braniselj, the DL deputy group leader, said Štefanec should think hard about whether he has any credibility left, but he refused to say whether Štefanec should resign.

The non-parliamentary Solidarity also urged Štefanec to resign. It said in a press release that party membership should not be a reason to deny anyone public office, but they resent Štefanec for "not disclosing his political affiliation to the members of the screening commission or the president."

The PS meanwhile declined to take responsibility for Štefanec's appointment, and instead attributed it to President Pahor, who the party said had been acquainted with all the facts.

The party said it had not nominated any candidate to the anti-graft body and had not been acquainted with any of its members biding for the post, while it also said party membership was not a legal obstacle to bidding for the post.

The controversy erupted after it was revealed that Štefanec was a member of the PS until the day he was picked by President Pahor. Both the screening commission and Pahor's office said they had not had knowledge about Štefanec's party affiliation.

Integriteta, the Slovenian branch of Transparency International, stopped short of calling for Štefanec's resignation but it questioned his decision to submit an application without clearly stating his party membership.

Credibility is the key to successful work of the anti-corruption commission. "Every candidate should realise that and reveal all facts that could give a different impression in the public...This credibility has evaporated," the NGO said in a statement.

Integriteta president Simona Habič was actually the chair of the screening commission, which said in a statement yesterday that membership of a party is not grounds for excluding applicants according to the act on integrity and prevention of corruption.