The Justice and Development Party (PJD), which has led a coalition governing the North African kingdom since late 2011, finds itself in a fight for re-election in October's vote as opponents take advantage of the rumours dogging the group.
Last week two vice presidents of the PJD's religious wing were suspended after the couple, both in their 60s, were found in a "sexual position" on a beach south of Rabat and arrested, local media reported.
It was, according to the Le360 news site considered close to circles in Morocco's royal court, "the cherry on the cake" of scandals involving the Islamists.
While the party itself is keeping a low profile, its backers accuse opponents in parliament and the media of conjuring a slur campaign to damage PJD credibility.
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"It is an old practice to defame and discredit the other (party) in the fight for power," historian Maati Monbij told AFP. "Some people at the heart of the state worry about the PJD coming first in elections."
Following years in opposition, the party found itself the head of a coalition in 2011 tasked with guiding Morocco through a turbulent period that saw other North African states convulsed by the Arab Spring uprisings.
That vote followed concessions from King Mohammed VI, the scion of a monarchy that has ruled the country for 350 years. A new constitution curbed some, but not all, of the king's near-absolute powers as autocratic regimes fell in Tunisia, Egypt and Libya.
Now, as a new election looms, the mounting controversy around the PJD provides a fillip for its rivals, most notably the Authenticity and Modernity Party (PAM). The list of accusations against the PJD is long and varied.
Last year, a woman filed a sexual harassment case against a PJD candidate near Marrakesh. A party member was arrested last month in the southwest of the country in possession of three tonnes of cannabis.
And a PJD governor is accused of using his political influence to try to force through the sale of 200 hectares (500 acres) of southeastern farmland. The latest furore caused by the in flagrante arrest of Omar Benhammad, 63, and Fatima Nejjar, 62, is all the more damaging to a party that extols Islamic moral behaviour.
Some social media users in Morocco have revelled in the case, posting videos of Nejjar in full Islamic headdress exhorting female students not to give in to "temptation and vice".
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Their case is a "tough lesson" for "a movement that calls people who go to festivals scoundrels, those who go out at night lewd, and that say men and women who mix will go to hell," the Al Ahdath daily wrote. "Do as I say not as I do," said a glib Huffington Post Morocco piece.
Akhir Saa, a newspaper close to the rival PAM party, went further, claiming that successive scandals were a "big blow for political Islam represented by the PJD." The PJD remains popular in the conservative country, despite limited success in tackling corruption, and is credited with lowering the budget deficit.
Supporters accuse the media and parties reportedly close to royal court circles of seeking to influence October's vote with negative PJD coverage. Cleric Ahmed Raissouni, who is close to the PJD, denounced what he called "police machinations". "The source of this aggression against the party today is made up of what I call the deep state," according to PJD lawmaker Abdelaziz Aftati.
Sociologist Mohammed Ennaji used Facebook to criticise what he termed "scheming and backstabbing" against the Islamic politicos. But others feel that the long charge sheet against the PJD exposes the double-standard governing the behaviour of Morocco's religious elite.
Huffington Post Morocco criticised what it called the party's "obsession and neurosis" over personal relationships, calling the beach affair a sign of its own "hypocrisy and frustrations".
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