"I am sure that Sangma would not be able to garner the support of even 100 out of the 508 legislators present in the Northeast," All India Congress Committee secretary in-charge Meghalaya, Sanjay Bapna told journalists.
However, Sangma had claimed that even some Congress MPs and legislators from the region would vote for him.
"I have already got feelers from them and their support is purely at a personal level as this is a secret ballot. I believe we should leave our political differences and stand united by supporting me in this election," he had told IANS.
Of the eight northeastern states, Assam, Meghalaya, Mizoram, Arunachal Pradesh and Manipur are ruled by the Congress.
The CPI-M rules Tripura, the Naga Peoples Front (NPF)-led Democratic Alliance of Nagaland (DAN) governs Nagaland while the Sikkim Democratic Front (SDF) is in power in Sikkim have extended their support to Mukherjee.
But powerful regional parties - the Asom Gana Parishad, Mizo National Front, United Democratic Party have extended their support to Sangma.
"His (Purno) appeal to one particular religion or section of society is not right," said Bapna.
"It is not right to divide the country on the basis of caste and community. Contribution towards society and the nation should be the only criteria (for the presidential poll). Public representatives do not get swayed on emotional lines. Legislators are quite judicious."
Criticising the expelled Congress leader and former Lok Sabha speaker for taking the support of BJP, Bapna said, "He (Sangma) is now taking the support of the BJP when he had all along opposed to the ideals of that party."
"The BJP just wants to put up a mask of secularism even though they know that Sangma is not going to win," he added.