Ennahda - or the Renaissance Party - was banned under the old dictatorship, its leaders forced into jail or exile.
Its dangerous struggle against the regime of the detested ousted President Zine al-Abidine Ben Ali partly explains its success at this election: Voters were looking for a clean break with the past and Ennahda's strong anti-corruption credentials proved a powerful electoral asset.
Ennahda remains an enigma though. It is an Islamist Party but does not seek an Islamic state.
It draws its values from religion but wants to be part of an essentially secular dialogue.
Its opponents are suspicious: Is it a wolf in sheep's clothing?
Will radical elements in the party use to democracy to gain power, only to use power to subvert democracy?
Is there a hidden agenda?
Islam is 'inspiration'
Rached Ghannouchi, the party leader, returned from two decades in exile in January.
His daughter, Intissar Ghannouchi is new to her own homeland - she was raised in London, studied law in Cambridge and the London School of Economics and specialises in human rights. She acts as her father's spokeswoman here.
"The inspiration for our values is Islam but we're concerned to address the modern daily concerns of Tunisians, within the context of modern culture.
"We are a political party, not a religious party," she told me, "just like the Christian Democrats in Germany.
Ennahda activists also speak of Turkey as the model to follow: A predominantly Muslim country, with a moderate Islamist government in a strictly secular republic.
"Will women be forced to wear the headscarf in the new Tunisia?" I asked her.
She politely batted the question away as though it were ridiculous.
"No-one will be forced to do anything. They are free to wear it or not wear it as they please."
"Will women who want to work be restricted to half days, as some scare stories have suggested?" I asked.
Again, the patient rebuttal: "No. In fact gender equality in the workplace - including equal pay for women - is part of our programme."
What is Ennahda's electoral appeal? Ask people here why they vote for the Islamists and they don't talk about religion.
They talk about honesty in public life, about the need for a government that doesn't steal from the people.
Ennahda won because more people believed they represented the most credible repudiation of the corruption and venality of the old dictatorship.
Where does that leave the secular and liberal opposition?
One leading opposition figure told me they would now have to work hard to provide a counterbalance to Ennahda, to keep the Islamists rooted in the secular democracy they say they believe in.
"There are many tendencies in Ennahda," Riadh Ben Fadhel, of the liberal Poll Democratique Moderne, told me.
"It is not a party, it is a movement. I am not worried about the current leadership. But there are radical elements that might one day seek to seize control of it," he said.
"The opposition parties must work hard. If we leave Ennahda alone, I think that the secular basis of our debate could quickly change to a debate of a religious vision of the state and that would be a very big catastrophe."
The 200-plus member constitutional assembly will now elect an interim government.
It will be a coalition, Islamist-led, in a secular democracy.
The assembly will also draw up a new constitution for the country.
It will have to decide whether it should seek a parliamentary or a presidential system; how to ensure the independence of the judiciary and the media; and how to elect its governments.
Tunisia is the second most-industrialised country in Africa, after South Africa.
It has a strong, educated, urban middle class, with a nuanced and clear understanding of how democracy functions.
Tunisians led the Arab Revolution with their revolution in January. Their dictator was removed in less than a month.
With this election, Tunisians have led again. For what is being born here is something new - a democracy in the Arab world.
There is much promise in that, and great optimism here.