The JHU, or National Heritage Party, said it wanted Rajapakse to honour a promise to rewrite the constitution and allow civil servants, judges, police and the elections chief to function independently.
“If the next election is held without carrying out the promised reforms, we will do our best to defeat President Mahinda Rajapakse,” JHU legislator and Buddhist monk Athuraliye Ratana told reporters.
Rajapakse came to power in 2005 promising to revert to a Westminster-style parliamentary democracy. But he secured a second term in 2010 and rewrote the constitution, removing the two-term limit on the top job.
Government ministers have said the president will seek election for a third term in January, two years ahead of schedule — attempting to secure another term before his party’s faltering popularity falls further.
The JHU has just three seats in the 225-member parliament but is considered influential among the country’s majority Buddhist community.
The party said it would drum up support for constitutional reforms by holding a public rally in Colombo on Wednesday.
“If the president sees a lot of public support for us, he may change the idea of a snap election,” Ratana said. “Our objective is to get him to deliver on the reforms he himself promised.”
The monk said his talks with Rajapakse’s Sri Lanka Freedom Party to remain in the ruling coalition ended in failure and the JHU had no option but to press for reforms through public meetings.
Rajapakse’s rivals have also raised doubts about whether he can legally seek a third term, arguing the amended constitution only applies to new presidents and cannot be used retroactively.
Rajapakse won popularity among Sri Lanka’s majority Sinhalese community in 2009 by crushing rebels who had waged a 37-year war for a separate homeland for ethnic minority Tamils.
But his party’s vote share plummeted at local elections in September, suffering its worst performance since Rajapakse first came to power nine years ago.
The president is also under intense international pressure to probe allegations that his troops killed up to 40,000 Tamil civilians while battling Tamil rebels in the final stages of the war.