The Madhesi People's Rights Forum (MPRF), the fourth largest party in the Constituent Assembly, broke their alliance with the CPN-Maoist to back, along with the CPN-UML, the Nepali Congress candidate Yadav, as the voting got underway in the 594-member House.
The Nepali Congress and CPN-UML also decided to support the candidature of MPRF candidate Paramananda Jha for the post of Vice President.
The three parties have a combined strength of 274 and with the support of other smaller parties they may cross the 300-mark in the assembly, analysts said.
"We will cross the magic number of 298 to elect the President," said Nepali Congress Vice President Gopalman Shrestha. "We have also received support from smaller parties."
B P Yadav, an MPRF leader, also said, "We have decided to back Nepali Congress candidate Ram Baran Yadav for President."
Earlier, Maoists's candidate Ram Raja Prasad Singh was expected to win the election after Madhesi parties had promised to back him.
The Nepali Congress and CPN-UML, the second and third largest parties respectively after Maoists, however, decided to forge an alliance with other groups, preventing the former rebels from getting the post of the first president of Nepal.
