Singh said the BSP supremo has proved herself against all odds and he can understand her "pain" when she was "humiliated" by Mulayam Singh Yadav.
Breaking his silence since Singh quit key party posts, the SP chief said nobody was indispensable.
"Kya Nehru nahin hain toh Congress nahin hai, kya main nahin rahoonga to SP nahin rahegi, SP barkarar rahegi (hasn't Congress survived without Nehru, wouldn't SP be alive without me, it would)," he told reporters in Hardoi.
Amar Singh, who has been taking potshots at his critics demanding his resignation from Rajya Sabha, said at a public meeting in Ghazipur that he would do so if Yadav returned him his kidneys he had sacrificed in the service of the party.
"To those who have been demanding my resignation from the Rajya Sabha, I have to say that if Mulayam Singh Yadav returns my kidneys I would resign," he said.
On Mayawati, he said in a television interview, "Now I can sense her pain of the guest house episode." He was recalling the incident in which Samajwadi Party workers had surrounded the Uttar Pradesh State Guest House in Lucknow in 1995 and roughed her up after BSP had withdrawn support to the Mulayam Singh Yadav government.
Stating that he was not without alternatives, Singh also heaped praise on Gandhi.
He told CNN-IBN that both Mayawati and Gandhi have "proved themselves against all odds."
"Both of them are political personalities in their own right. One lady has proved herself in the state of UP against all odds and the other has proved herself in the country against all odds," he said.
However, Congress took a jibe at Singh's remark that he can understand Mayawati's "pain" saying it took him so long to understand it.
AICC general secretary in-charge for Uttar Pradesh Digvijay Singh, at the same time, made it clear that if Singh wants to join Congress, the initiative should come from him.
On whether Amar Singh is warming up to BSP because Congress is not welcoming him, the AICC leader said, "There is no question of welcoming or not welcoming Amar Singh ji. There is a process for it in the party."
Reacting to Singh's statement, SP spokesman and general secretary Mohan Singh said he is praising Mayawati and Gandhi because he has run out of options.