For the third time in just two months, Tunisian voters will head to the polls this weekend, this time to elect a president in a runoff vote that presents a stark choice between the country's past and its Arab Spring revolution.

Tunisians must decide Sunday between Beji Caid Essebsi, an 88-year-old veteran of previous regimes, and Moncef Marzouki, a 75-year-old human rights activist who is the outgoing interim president. The two candidates have been at each other's throats for the past two weeks of campaigning.

Under Tunisia's new constitution, the president will be responsible for security, defense and foreign affairs. Analysts said both candidates have run negative campaigns emphasizing what dangers a victory for their opponent would bring.

"Both of them center their campaigns around fear," said Kais Saied, who teaches constitutional law at Tunis University.

The vote comes days after the fourth anniversary of the self-immolation of Mohammed Bouazizi, an itinerant fruit seller who killed himself to protest police actions in the town of Sidi Bouzid. His death sparked an uprising that overthrew Tunisia's longtime dictator, Zine El Abidine Ben Ali, and spread to neighboring countries.

Sunday's election is largely about the legacy of that revolution, which ushered in a period of economic turmoil and political instability marked by the temporary rise of Islamists to power.

In October's parliamentary elections, many Tunisians turned away from a moderate Islamist party, Ennahda, and backed Essebsi's Nida Tunis party, a loose coalition of old-regime politicians and trade unionists promising stability and a return to prosperity.

Essebsi received 39.5 percent of the vote to lead last month's first round that featured 27 candidates. Should he triumph Sunday, his party will control all levers of government. Marzouki came second with 33.4 percent. Should he win, it would set up an inevitable collision between the presidency and parliament.

Essebsi has campaigned on restoring what he calls the "prestige of the state" and evokes the legacy of Habib Bourguiba, modern Tunisia's founding father, who promoted the middle class and education — and tolerated little opposition.

He paints Marzouki's three-year tenure as a disaster, accusing him of opening the country to terrorist attacks. He notes Marzouki's support from Islamists in Ennahda, which finished second in October's parliamentary elections.

Marzouki in turn warns that Essebsi would return Tunisia to Ben Ali-style authoritarian rule. He has hinted, too, that 88 is too old to rule.

Support for the candidates is split by geography.

The wealthier north and coastal regions back Essebsi, who is from the coastal Sahel that traditionally has produced Tunisia's ruling class. The relatively impoverished south and interior regions broadly support Marzouki and his message of change.

Ennahda, a powerful force despite its second-place showing in October, is officially neutral. But its supporters are expected to vote heavily for Marzouki out of fear of the dictatorial past, when they were imprisoned by the thousands or, like Marzouki, forced into exile.

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Associated Press reporter Ben Wiacek in Tunis contributed to this report.