Want to see the first successful Arab democracy in action? Tune in Sunday, when tiny Tunisia will hold its first legislative elections since the ratification of its liberal-democratic constitution in January. Tunisia is where the Arab Spring began in 2011, and it’s just about the only place where that movement for freedom and democracy hasn’t failed. The complex politics of these elections will tell us a lot about whether Tunisia is going to mature into a functioning democracy — or revert to dictatorship like Egypt.

The protagonists in the legislative election are really just two, and they’re both highly interesting. The first is Ennahda (it means the Renaissance), a party that advocates both Islamic values and egalitarian democracy. Ennahda’s origins lie in the international Muslim Brotherhood. Under the leadership of Rashid Ghannouchi, the party has established itself as the most politically moderate Islamic political actor anywhere in the Arabic speaking world.

In the last elections, Ennahda won a plurality in the constituent assembly that governed Tunisia during its extended period of transition. In that role, Ennahda formed coalitions with secular parties of the right- and left-center. It shared power with those parties, shakily at times but ultimately peacefully. It dropped any mention of Shariah from the constitutional draft. And perhaps most remarkably, Ennahda has announced that it will not run a candidate in November’s presidential elections. If it keeps to that promise, the legislative elections are the party’s only shot to remain a predominant force in Tunisian politics.

The other party seen as a leading contender in the legislative elections didn’t exist the last time Tunisians voted. Nidaa Tounes, “the Call for Tunisia,” hopes to represent a broad base of secular Tunisians. Striving to avoid internecine conflicts that have doomed most of Tunisia’s secular parties to irrelevance over the last three years, Nidaa has avoided making too many specific policy promises. To many voters, the party’s biggest appeal is that it isn’t Ennahda.

Nidaa’s leader, Beji Caid Essebsi, is a throwback to an earlier era of Tunisian politics — literally. Now 87, he served in the government of Habib Bourguiba, the revolutionary hero of independence from France, from 1957 to 1971. Essebsi was an ambassador and legislator under dictator Zine El Abidine Ben Ali. After Ben Ali’s ouster and before the first elected coalition government could take office, Essebsi served honorably for most of a year as interim prime minister, after which he peacefully stepped down. He intends to run for president in November.

At a personal level, the differences between Essebsi and Ghannouchi could hardly be more marked. I met Essebsi in his rather grand house a couple of years ago, and his political rhetoric reminded me of the newspaper articles that my teachers used for Arabic lessons when I was a teenager in the 1980s. Even then the nostrums sounded hopelessly outdated. Essebsi was urbane, worldly and obviously highly intelligent. But I couldn’t escape the feeling that I was speaking to a political dinosaur.

Ghannouchi, soft-spoken to the point of diffidence, spent a good chunk of the 1980s in jail. He passed the next two decades in exile in Paris and London, where he developed his theories of an Islam compatible with egalitarian democracy. Essentially an intellectual, Ghannouchi is the leader of Ennahda, but has never held elected office and shows no interest in doing so. He interrupted one of our conversations to pray quietly and briefly on a mat in his modest office in Ennahda’s worryingly unprotected headquarters. Whatever Ghannouchi may be — and his many opponents condemn his apparent moderation as an act — he differs drastically from the traditional model of the modern Arab politician from the era of dictatorship.

What will happen Sunday? Polls are relatively unreliable, but in general they have the two parties running close with Nidaa perhaps somewhat ahead. For Ennahda, the best result would be to win a plurality, then form a governing coalition with Nidaa or smaller secularist parties. So far, the strategy of coalition of compromise has paid off for the Islamic democrats. Unlike the Egyptian branch of the Muslim Brotherhood, Ennahda has avoided threatening or alienating secular forces in the society to the point where they seek its violent suppression. If Ennahda does win a plurality, expect the party to keep its promise of not running a presidential candidate. Ennahda knows that with a legislative plurality and the president from his own party, it would be too powerful and might well provoke a response.

If Nidaa wins a plurality, however, the situation will become more complicated. Nidaa might well believe that it could form a coalition without Ennahda. Nidaa would have significant momentum to win the presidential contest — at which point Essebsi would find himself an 88-year-old with a serious secular mandate. The temptation to use undemocratic means and get rid of Ennahda as a viable political force could be hard to resist. The result would be a disaster for Tunisia’s hopes of becoming a functioning democracy.

Faced with this danger, elements within Ennahda may reason that the party should break its pledge and run its own presidential candidate if it loses in the parliamentary elections. The theory would be self-protection. Ennahda stalwarts might also reason that a president associated with the Islamic democratic movement would be less threatening if he served alongside a secularist prime minister from Nidaa Tounes.

Either way, Tunisia will face challenges to making its democracy work. The key to success is for all sides to realize that winner-take-all politics are incompatible with democratic development. So vote early and often, and hope for coalition government. Arab democracy needs all the help it can get.

[Cross-posted at Bloomberg View]

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Noah Feldman , a professor of constitutional and international law at Harvard, is a Bloomberg View columnist.