Nana Konadu Agyeman Rawlings has told the BBC that her husband had lost authority in the National Democratic Congress (NDC) and merely fulfilled his "constitutional responsibility" as founder of the party by attending its events.
Some NDC members had used him to rally support during elections and then ditched him, she said.
"The NDC was founded on certain values which they [party leaders] have thrown out of the window so the NDP is taking those values to make sure there is transparency in governance," Mrs Rawlings said.
She said she was running for president because she was worried about worsening corruption and mismanagement, and wanted to be at the "centre" of government so that she could improve the lives of people.
When Mr. Rawlings addressed the National Democratic Party (NDP) convention at the weekend, he said the party would "restore hope and the moral values that the country needs to move forward".
At the same event, he also praised Mr Mahama, who became president earlier this year following the death of his predecessor, John Atta Mills.
"Thank God John Mahama has taken over and agreed that he will restore hope in the NDC… Should he win re-election, then we can join hands again," he said. Our correspondent says NDC leaders are reportedly meeting to discuss Mr Rawlings' presence at the NDP's congress.
There is concern that the NDC could be weakened by Mrs Rawlings' decision to fight the election under the new party's banner, he adds.
Mrs Rawlings challenged Mr Mills last year in a bid to get the NDC's nomination, but gained only 3.1% of the vote.
