Brussels-based International Crisis Group (ICG) says in a report that for the first time since President Mahinda Rajapaksa won the war against the LTTE militant group in 2009, "it can no longer be taken for granted that Rajapaksa and with him his powerful brothers and other family members will remain in power indefinitely".
ICG said for the first time in years the opposition together with critical voices among Sri Lanka's beleaguered civil society are sensing that political change is a real possibility.
ICG said Sirisena seems set to pose a strong challenge to Rajapaksa as a number of key ruling party members joined him.
A deputy minister and at least three other lawmakers have also left Rajapaksa's ruling coalition United People's Freedom Alliance.
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Earlier in November, the National Heritage Party (JHU) announced it was exiting the government. As the main party of Buddhist monks, its departure could damage Rajapaksa's support in a country where Buddhists comprise nearly 70 per cent of its population of 21.8 million.
Rajapaksa called a snap election to seek a third consecutive term in office, two years ahead of schedule.