The election will be held a month after the first meeting of the 601-member body that will draft a new Constitution aimed at ending political unrest in Nepal.
According to the provisions of the seven-point agreement that paved the way for the formation of a government under Nepali Congress (NC) president Sushil Koirala, the party today directed its lawmakers to vote for the CPN-UML candidate.
The NC, the largest party in the Constituent Assembly with 194 seats, issued a whip to all its lawmakers to be present for election tomorrow, according to a statement issued by the party.
The CPN-UML, the second largest party with 173 lawmakers, had earlier decided not to join the cabinet after it was denied the important home affairs portfolio in the coalition government.
Koirala was elected Prime Minister on February 10 with 405 votes in his favour, ending months of political uncertainty after the Constituent Assembly polls held on November 19.
The Assembly's chairman also serves as Speaker of the House.