Tunisia's Islamist Ennahda Party became the biggest in parliament after more lawmakers in President Beji Caid Essebsi's Nidaa Tounes party resigned on Monday over the role of his son, saying they feared a return of the hereditary transfer of power.
The rift does not present an immediate threat to the coalition government, which includes Ennahda, but it comes at a delicate time as the North African state struggles to contain jihadist violence and encourage economic growth.
With a new constitution and free elections, Tunisia has been praised as a model of democratic transition since the ouster of Zine Abidine Ben Ali and has mostly escaped the violent upheaval seen in other countries in the 2011 Arab Spring uprisings.
Divisions have been growing inside Nidaa Tounes, a secular party formed after the 2011 revolt, since a dispute emerged last year between a wing of the party led by the president's son, Hafedh Caid Essebsi, and another led by Mohsen Marzouk, one of its founders.
After the first resignations last week, two more lawmakers said they were resigning from Nidaa Tounes on Monday, bringing the total to 19. That leaves the party with 67 lawmakers in the 217-member congress, while Ennahda has 69.
Those who have resigned, including Marzouk, say they will form a new party.
They said their fears Hafedh Caid Essebsi was seeking control of the party were reinforced on Sunday when he was appointed to its central committee as legal representative and general secretary. They have denounced what some see as a return of the autocratic style of the Ben Ali era.
Essebsi's backers have dismissed claims of a dynastic handover.
Nidaa Tounes emerged as a political force in 2013 to lead protests against a government formed by Ennahda. It beat Ennahda in the 2014 election and went on to form a coalition with its rival.
The resignations may complicate attempts to push through sensitive reforms that Tunisia's international lenders are demanding to curb public spending and kickstart an economy hit by three major Islamist militant attacks last year.