Makkal Needhi Maiam vice president R Mahendran on Thursday resigned from the party, saying "there seems to be no democracy" in the Kamal Haasan-led organization.
The actor-politician later declared him a "traitor".
Mr Haasan said "a weed" had weeded itself out from the MNM. He also said that the leader was about to be sacked from the party.
The decision of Mr Mahendran, the party's second-ranked leader, came on a day when MNM announced that seven other senior functionaries have also tendered their "resignations."
Mr Mahendran, who unsuccessfully contested the April 6 Assembly polls from Singanallur constituency in Coimbatore, told reporters he submitted a letter to Mr Haasan, informing him about his resignation from the party post as well as its primary membership.
He blamed a "few advisors" at "the top" over the running of the party and said Mr Haasan's way of steering the party "is not okay."
Nr Mahendran said he "felt like there is no democracy."
"Many worked hard with the hope that this will change after polls. So I stayed with him (Haasan) but felt that in the last month or so there was no change in his approach."
He said he came armed with his resignation letter to the party's Executive Committee meeting today, held in the backdrop of the party's debacle in the assembly polls, and handed it over to Mr Haasan.
Meanwhile, the party in an official statement said that besides Mr Mahendran, other seniors including AG Maurya, M Muruganandam, CK Kumaravel and Umadevi "gave their resignation letters."
Mr Haasan will soon consider them, it said.
"We saw that there were traitors along with enemies in the field. There was a unanimous chorus for weeding out traitors. On top of the list was Dr R Mahendran," Mr Haasan said.
While his party colleague had alleged "there is no democracy" in the party, "he is the biggest example that even democracy will lose at times," the MNM chief added.
He accused Mr Mahendran of trying to blame others for his "dishonesty and inefficiency".
"He cleverly stepped away understanding that he would be sacked. I am happy like you that a weed has understood it is a weed and has weeded itself out," he said.
"We are never bothered about the cowards who run away during a defeat. There shall be no slightest deviation from our ideology-driven path," the party founder added.