Udaya Gammanpila from the Buddhist nationalist National Heritage Party defected from the opposition to become the third leading politician to support Rajapaksa in the election.
Rajapaksa faces his former health minister Maithripala Sirisena who defected to the opposition to become the common candidate of the opposition for the next presidential election.
Sirisena led a revolt in Rajapaksa's ruling coalition, taking with him 11 ministers and lawmakers. The contest had picked up momentum after his defection.
But in the first major counter move, the opposition United National Party's general secretary Tissa Attanayake defected to Rajapaksa's ruling coalition on Monday.
Jayantha Ketagoda, deputy leader of the Democratic Party also joined him on Monday. Attanayake was sworn in as the Minister of Health on Thursday.

Rajapaksa called the election two years ahead of schedule in an apparent attempt to seek a fresh mandate before his party's popularity tumbles further, after dropping over 21 percent in September's local elections. Some 14 million people are eligible to vote at the country's election.

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