COLOMBO (Reuters) - Sri Lankan President Mahinda Rajapaksa on Thursday drew closer to the two-thirds parliamentary majority he needs to change the constitution after two opposition legislators joined his ruling alliance.

The president's United Peoples Freedom Alliance won a sweeping majority at April polls after Rajapaksa's re-election three months earlier.

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Rajapaksa has already struck a tentative deal to return Sri Lanka to rule by an executive prime minister. That would allow him to bypass the existing two-term limit as head of state and run the country as prime minister.

"Two opposition members of parliament from different political parties joined the government today, taking the strength of the ruling alliance to 146 in the 225-member parliament," said Lucien Rajakarunanayake, a presidential spokesman.

The crossovers come as the government and the main opposition United National Party are in discussions to change the constitution and reduce the president's executive powers.

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Critics blame the president's strong executive powers under the present constitution for political interference in the judiciary, public service and police, allowing the head of state to manipulate the system as he or she wishes.

(Reporting by Ranga Sirilal; Editing by Bryson Hull and Ron Popeski)