All Sherry Rehman wants is to go out – for a coffee, a stroll, lunch, anything. But that's not possible. Death threats flood her email inbox and mobile phone; armed police are squatted at the gate of her Karachi mansion; government ministers advise her to flee.

"I get two types of advice about leaving," says the steely politician. "One from concerned friends, the other from those who want me out so I'll stop making trouble. But I'm going nowhere." She pauses, then adds quietly: "At least for now."

It's been almost three weeks since Punjab governor Salmaan Taseer was gunned down outside an Islamabad cafe. As the country plunged into crisis, Rehman became a prisoner in her own home. Having championed the same issue that caused Taseer's death – reform of Pakistan's draconian blasphemy laws – she is, by popular consensus, next on the extremists' list.

Giant rallies against blasphemy reform have swelled the streets of Karachi, where clerics use her name. There are allegations that a cleric in a local mosque, barely five minutes' drive away, has branded her an "infidel" deserving of death. In the Punjabi city of Multan last week opponents tried to file blasphemy charges against her – raising the absurd possibility of Rehman, a national politician, facing a possible death sentence. "My inbox is inundated. The good news is that a lot of it is no longer hate mail," she says with a grim smile. "But a lot of it is."

Pakistani politicians have a long tradition of self-imposed exile but 50-year-old Rehman – a former confidante of Benazir Bhutto, and known for her glamour, principled politics and sharp tongue – is surely the first to undergo self-imposed house arrest. Hers is a luxury cell near the Karachi shore, filled with fine furniture and expensive art, but a stifling one. Government officials insist on 48 hours' notice before putting a foot outside. Plots are afoot, they warn.

She welcomes a stream of visitors – well-educated, English-speaking people from the slim elite. But Pakistan's left is divided and outnumbered. Supporters squabble over whether they should call themselves "liberals", and while candle-lit vigils in upmarket shopping areas may attract 200 well-heeled protesters, the religious parties can turn out 40,000 people, all shouting support for Mumtaz Qadri, the fanatical policeman who shot Taseer. "Pakistan is one of the first examples of a fascist, faith-based dystopia," warns commentator Nadeem Farooq Paracha.

Is it really that bad? At Friday lunchtime worshippers streamed into the Aram Bagh mosque, a beautiful structure in central Karachi inscribed with poetry praising the prophet Muhammad. "He dispelled darkness with his beauty," read one line. At the gate a banner hung by the Jamaat-e-Islami religious party offered less inspiring verse: "Death to those who conspire against the blasphemy laws."

Qamar Ahmed, a 50-year-old jeweller, said he "saluted" Taseer's killer, Qadri. "Nobody should insult the glory of the prophet, who taught us Muslims to pray," he said.

A sense of siege is setting in among Pakistan's elite. Hours later, at an upscale drinks party in the city, businessmen and their wives sipped wine and gossiped about second homes in Dubai. One woman admitted she wasn't aware of Rehman's plight because she had stopped reading the papers. "Too much bad news," she said.

Yet Pakistan is not on the verge of becoming a totalitarian religious state. The fervour is being whipped up by the normally fractious religious parties, delighted at having found a uniting issue. Leading the protests is Jamaat-e-Islami, which made the mistake of boycotting the last election and now wants to trigger a fresh poll.

More significant is the lack of resistance from every other party. Rehman is polite when asked about the silence of her colleag ues in the ruling Pakistan Peoples party on the blasphemy issue. "They feel they want to address this issue at another time," she says. The truth is, they have abandoned her.

The party played with fire over the blasphemy issue last November when President Asif Ali Zardari floated the idea of a pardon for Aasia Bibi, a Christian woman sentenced to death on dubious blasphemy charges. According to Rehman, he also agreed to reform the law. But then conservative elements in the party objected, a conservative judge blocked the pardon and, even before Taseer had been killed, the party had vowed not to touch a law that has become the virtual sacred writ of Pakistani politics.

The opposition has also been quiet. "The greater the failure of the ruling class, the louder the voice of the cleric," says politician and journalist Ayaz Amir.

The mess is also the product of dangerous spy games by the powerful army, which propped up jihadi groups for decades to fight in Afghanistan and India. Some of those militants have now "gone rogue" and allied with al-Qaida; others, according to US assessments in the WikiLeaks files, are still quietly supported by the military. "Our establishment, especially the army, is in league with these people," says Javed Ahmad Ghamidi, a moderate cleric. "And until they stop supporting them they will never be weakened."

The furore has exposed the fallacy of western ideas about "moderate" Islam. Qadri is a member of the mainstream Barelvi sect, whose leaders previously condemned the Taliban. But after Taseer's death, Barelvi clerics were the first to declare that anyone who even mourned with his grieving family was guilty of blasphemy.

Progressives demonstrate loudly in the English press and on Twitter but lack political support, having largely spurned corruption-ridden politics. Politicians say now is the time to come back. "They will be contemptuous of the politician, but they will not actually soil their hands with politics. But none of them has a constituency from which to stand," says Amir.

And there are signs that extremists do back down when confronted. Qari Munir Shakir, the cleric accused of calling Rehman an "infidel", denied his comments after Rehman supporters filed a police case against him. "It's all been blown out of proportion," he said. "All I did was ask her to take the law back. I can't imagine calling her a non-Muslim or declare her Wajib ul Qatil [deserving of death]."

Rehman is unlikely to attend Pakistan's parliament when it resumes this week. Her progressive credentials are strong, having previously introduced legislation that blunted anti-women laws and criminalised sexual harassment. But critics, including senior human rights officials, say she made a tactical mistake in prematurely introducing last November's blasphemy bill without the requisite political support.

"There's never a right time," she retorts. "Blasphemy cases are continually popping up, more horror stories from the ground. How do you ignore them?" At any rate the bill is a dead letter: clerics are demanding its immediate withdrawal from parliament and the government is likely to comply.

Amid the gloom there is some hope, from unlikely quarters. On a popular talk show last Friday night Veena Malik, an actress who faced conservative censure for appearing on the Indian version of Big Brother, gave an unforgettable tongue-lashing to a cleric who had been criticising her. "You are attacking me because I am a soft target," she railed into the camera, wagging her finger.

"But there's a lot more you can fix in the name of Islam… What about those mullahs who rape the same boys that they teach in mosques?" As the mullah replied, she started to barrack him again.

Hope also springs inside the silent majority. "The blasphemy law should be changed," declared Muhammad Usman after Friday prayers. Clutching his motorbike helmet, the 30-year-old pharmaceutical company representative said he was unafraid of speaking his mind. "It's just the illiterate ones who are supporting Mumtaz Qadri. They don't have any real religious knowledge," he said.

Some analysts downplay the worst predictions, saying blasphemy is exceptionally sensitive in a country obsessed by religion. They are right. Pakistan will soon return to more concrete worries: Taliban insurgents, economic collapse, the rise of extremism. Yet there is no doubt the aftermath of Taseer's death points to a country headed down a dangerous path.

"We know from history that appeasement doesn't pay. It only emboldens them," said Rehman.

She has no idea how long her self-imposed house arrest will last, but the precedents are ominous. In 1997 a judge who acquitted two Christians accused of blasphemy was gunned down – three years after the judgment.

"It makes me realise that life is pretty fragile," she says. "But we don't want to leave. I see no meaning to a life away from my country. It's my identity, it's everything."