Opposition forces have struggled to carve out an identity ever since Tunisia's post-revolution elections brought Islamists to power at a time of disunity between secular parties.
The assassination on Wednesday of leftist opposition leader Chokri Belaid has thrown up an opportunity to close ranks but it "may be seized or squandered," said Kamel Laabidi, founder of Vigilance for Democracy and Civil State, an NGO.
The only joint initiative adopted by the secular parties has been an indefinite boycott of the National Constituent Assembly.
The opposition has also come out in support of Prime Minister Hamadi Jebali's initiative to form a new government of technocrats, a plan which has angered supporters of his own ruling Islamist party Ennahda.
Jebali proposed his initiative at a time of political crisis triggered by the killing of Belaid, a harsh government critic, that has inflamed unrest across the North African country.
Tunisia's opposition has been undermined by differences ever since the 2011 revolution that ousted former dictator Zine El Abidine Ben Ali.
"Decades of authoritarianism and repression fragmented the democrats and prevented the emergence of a united front, bringing the Islamist Ennahda to power," said Laabidi.
Such divisions and personal ambitions led to the defeat of secular groups in October 2011 elections, the first free vote after the fall of Ben Ali, leaving the door open to Ennahda which won 89 seats in the 271-member parliament.
"How does one put pieces together in a camp where divided leaders are prisoners of their personal ambitions and political calculations?" asked Laabidi, who was forced into exile under the Ben Ali regime.
A Democratic Group, which has 35 seats in the constituent assembly, has a common line of resistance to the Islamists but has failed to come up with clear ideas of its own.
Its economic policies remain vague, even though unemployment and poverty were at the heart of the revolution which ousted Ben Ali.
Political analyst Ahmed Manai said the opposition groups have struck up alliances to resist Ennahda but failed to come up with a credible programme of government.
At the centre is Beji Caid Essebsi, an 86-year-old ex-premier who was also a minister under the father of Tunisia's independence, Habib Bourguiba.
A Unity for Tunisia sealed at the end of January between Essebsi's Nidaa Tounes and Al-Massar, made up of communists and leftists, also groups Al-Joumhouri, composed of liberals and centrists.
The leftist Popular Front, founded last October and in which Belaid was a leader, unites a dozen Marxist and pan-Arab groups.
Other former opponents of Ben Ali have joined forces with Ennahda, such as Moncef Marzouki who became president after he returned from exile following the fall of Ben Ali.
Many democrats have been "disappointed to see him accept their orders and reduced to impotence in the face of Islamists," said Laabidi.
Mustapha Ben Jaafar, a social democrat, has also lined up with Ennahda to become parliament speaker, in the process losing the support of activists and of half the MPs of his party.