Spokesman Wijayananda Herath said the Ceylon Workers' Congress (CWC) leader Armugam Thondaman met Rajapakas this morning to pledge support.
The CWC is an ally of Rajapaksa's UPFA coalition. They have a history of working in coalition with the main opposition UNP too. The party's support would be crucial for Rajapaksa to win a third term.
The Sri Lankan President is expected to call a presidential election a year ahead of the schedule in January 2015. The election date is to be officially announced in late November. Rajapaksa draws his strength from the 74 per cent Sinhala majority community.
They view him as a hero for defeating the LTTE's 30-year-old campaign for a separate Tamil minority state in the north and east regions.