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The former President, who is expected to announce his bid to contest the 2020 election in due course, said many National Democratic Congress members did not turn out to vote in the party's stronghold.
Mr Mahama lost the polls to Mr Akufo-Addo by more than one million votes, becoming the first incumbent president to lose an election.
He said: “NPP did not win that election; we[NDC] lost it because apathy was created in our ranks. Many of our people said they will not come out and vote.”
“If you take the 2016 elections, we lost more than 1 million votes. It means that many of our supporters refused to come out and vote.
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"I want to apologize to everybody to forget about what anybody did to you. It might have been your party Chairman, your party secretary. It might have been me. I want to apologize.”