According to him, disunity among the rank and file of the NDC was a major cause of the defeat in 2016.
He made this known when he met with all the six other aspirants in the party's presidential primary.
Prof. Joshua Alabi, Ekwow Spio-Gabrah, Sylvester Mensah, Nurudeen Iddrisu, Alban Bagbin and Goosie Tanoh all contested the party's flagbearer position and have pledged to ensure the NDC win power in 2020.
Addressing the aspirants, Mahama said "One of our attitudes going into 2016 is a certain lack of unity. We didn't go in together as one. So, I think this initiative will smoothen over any issues and differences and moving together as one into the battle of 2020 enhances our opportunities and chances of clinching the victory for our great party."
He called on them to channel their energies towards the common goal of taking back power.
"I believe that one of our Achilles hills going into 2016 [election] was a certain lack of unity…and so I think this initiative will smoothen over any issues and differences and moving together as one into the battle of 2020 enhances our opportunity and chances of clinching the victory for our great party," he added.
Mahama polled 213, 487 representing 95.23 percent of the total votes cast to topple the six other contestants. A former vice chancellor of UPSA, Prof. Joshua Alabi, came in a distant second with 3,404 votes representing 1.52% of total votes cast and was followed by second deputy speaker of Parliament Alban Bagbin who had 2,301 votes representing 1.03% of the total votes.
Goosie Tanoh obtained 2,091 votes (0.93%), Sylvester Mensah, formerly with the NHIA polled a paltry 934 (0.42%) votes with former Trade Minister Ekow Spio-Garbrah garnering 1,447 votes representing 0.65%.
Nurudeen Iddrisu came last with 520 votes representing 0.23% of the total votes cast.
The aspirants also said the election is over and it behooves on every NDC member to get behind the flagbearer for an emphatic victory in 2020.