Nisa Ariyani staunchly supported Indonesia's Muslim parties her whole life, throughout decades of authoritarian rule and at the three legislative elections after the country became democratic.

But when tens of millions vote in parliamentary polls in the world's most populous Muslim-majority country on April 9, the 42-year-old teacher is set to join a growing number who will not cast their ballot for an Islamic party.

Indonesia's five main Muslim parties are heading for their worst ever showing at the elections, hit by explosive scandals and a growing trend among voters not to pick parties purely on religious grounds.

"I have lost my faith in Islamic parties, and I will vote for nobody," said Ariyani, who lives in the capital Jakarta and has worn a headscarf all her life, even during the long rule of dictator Suharto when it was uncommon in Indonesia.

Her change of heart is due to a specific case -- a sordid scandal involving clandestine hotel room sex and huge kickbacks that rocked the party she had supported at previous elections, the Prosperous Justice Party (PKS).

Before supporting the PKS, Indonesia's biggest Islamic party, she backed the United Development Party, one of the few opposition groups allowed by Suharto and still around now.

Paradoxical shift?

The decline in support for Muslim parties -- which range from moderate groups to more extreme ones that want to introduce Islamic sharia law -- since the end of authoritarian rule in 1998 seems at first glance a paradox, analysts say.

Since the downfall of Suharto, who backed a secular state and was against the strong influence of Islam in public life, Indonesia has appeared to have become more Islamic, not less.

An increasing number of women wear the headscarf, Islam-influenced goods -- from fashion brands to apps that remind you when to pray -- are all the rage, while some people have even chosen to live in strict Islamic communities, rejecting the trappings of modern life.

The tumultuous years following the end of Suharto's regime were also accompanied by an upsurge in Islamic extremism, notably the 2002 Bali bombings, in which 202 people -- mostly foreign tourists -- were killed.

A crackdown over the past decade has weakened the most dangerous groups but Islamic extremists still regularly target domestic security forces.

Despite this, the country's five main Islamic parties -- among 12 running in the parliamentary elections -- have seen their popularity slide in the sprawling archipelago nation where more than 90 percent of the population is Muslim.

Their combined share of the vote fell to around 26 percent at the 2009 legislative polls from around 34 percent a decade earlier.

Dodi Ambardi, a director at the Indonesian Survey Institute, predicts their support will fall to only 15 percent in the coming elections, particularly due to a drop for the PKS, which won almost eight percent in 2009.

No Islamic party is expected to do well enough to put a candidate forward for the presidential polls in July. A party or coalition of parties must win 20 percent of the seats in parliament or 25 percent of the national vote at the April elections to do so.

Savvy voters

But growing sophistication among voters who no longer simply focus on religion is a greater factor in the parties' declining support, analysts say.

"In choosing which party they will vote for, Muslim voters no longer think of their religion but rather the party's track record and policies," Ambardi told AFP.

In this regard, Islamic parties have notably failed.

They have not developed into well-run organisations and have relied on the mistaken belief that pious Muslims would vote for them regardless, said Noorhaidi Hasan, an expert in Islam and politics.

"Islamic parties are too ideological, offering an Islamic ideology but no other action," said Hasan, from Sunan Kalijaga Islamic university in Yogyakarta, on the main island of Java.

He also said increasing signs of Islam in everyday life did not necessarily mean people were becoming more Islamic and would automatically vote for Muslim parties -- simply that they were now free to express their faith publicly and they had the financial means to do so.

"Middle-class Muslims are not expressing their religion for the sake of religion -- but for social status and lifestyle," he said.

Despite their travails, Muslim parties are still likely to attract some support and the PKS is convinced many have already forgotten last year's controversy, which saw its ex-president jailed for 16 years.

"Support for the PKS is like a pillow -- once a burden is lifted, it will return to its normal shape quickly and easily," said party spokesman Dedi Supriadi.

And Islamic parties could still remain influential by providing support to the bigger parties, observers believe.

Coalition governments are the norm under Indonesia's complex electoral system, and there are currently four Islamic parties in the six-party coalition of President Susilo Bambang Yudhoyono.