The regional party had applied for merging with the Congress in January last and the party was merged with the national party last evening, Congress sources said.
MSCP President Y Mangi said his party could not do much for the state because of having a few MLAs in the assembly and that was the reason for merging with the national party.
The strength of Congress now rose to 47 in the state assembly, with Trinamool Congress seven, Naga People's Front (NPF) four, National Congress Party (NCP) one and Lok Janasakti Party (LJP) one.