Ripun Bora had accused several Congress leaders of working with the BJP.
The Assam Congress on Monday hit out at former state unit chief Ripun Bora for "showing selfish nature", a day after he quit the party, accusing several of its leaders of working for the BJP.
Congress Legislative Party leader Debabrata Saikia came down heavily on Mr Bora and said that instead of helping the party which is at a critical juncture, the former MP has shown his "selfish nature" for his personal interests.
Mr Saikia said that it was the Congress that helped Mr Bora shape his political career, and it was only unfair on his part to leave the party, particular at a time when it needs veteran leaders to stay united.
Ripun Bora, in his resignation letter addressed to Congress president Sonia Gandhi, cited infighting within the Assam Congress as the primary reason for his desertion.
Ending his four-decade-old relationship with the party, he joined the Mamata Banerjee-led Trinamool Congress on Sunday.
He also blamed party leaders for his defeat in the Rajya Sabha polls and even accused a section of Assam Pradesh Congress leaders of maintaining a secret understanding with the BJP government, mainly Chief Minister Himanta Biswa Sarma.
His move comes around a fortnight after alleged cross-voting in the Rajya Sabha election cost his seat in the Upper House of parliament.
Assam Chief Minister Himanta Biswa Sarma said it was natural for Mr Bora to resign from the Congress after MLAs of his own party "worked for his defeat" in the Rajya Sabha election held on March 31.
The 66-year-old leader, who has been a Congress member since his student days in 1976, joined the Trinamool in Kolkata in the presence of the party's national general secretary Abhishek Banerjee, Mamata Banerjee's nephew.
Rajya Sabha MP Sushmita Dev, who had switched over to the Trinamool from the Congress in Assam last year, was also present at the ceremony.