92-year old Bardhan, who lives in the CPI Headquarters here, felt uneasy around 8:00 am and then lost consciousness.
He was immediately rushed to the G B Pant Hospital in central Delhi, CPI leader Atul Kumar Anjaan said.
He described Bardhan's condition as "critical" and said the former General Secretary of the Communist Party of India was admitted in the emergency wing and being taken care of by senior doctors.
Bardhan's wife, a professor in Nagpur University, died in 1986. The couple has a son and a daughter.
Ardhendu Bhushan Bardhan was a leading figure of the trade union movement and Left politics in Maharashtra and had won as an independent candidate in the Maharashtra Assembly in 1957.
He later rose to become the General Secretary and then the President of the All India Trade Union Congress, the oldest trade union in India.
Bardhan moved to Delhi politics in the 1990s and became the deputy general secretary of the CPI. He succeeded Indrajit Gupta as the General Secretary of the party in 1996.