Mr Natarajan is admitted at a hospital in Chennai and is expected to undergo a liver transplant. The 74-year-old, who has suffered multiple organ failure, has reportedly asked to see his wife.
VK Sasikala, long-time aide to J Jayalalithaa, seized control of the AIADMK shortly after the latter's death in December last year as Tamil Nadu Chief Minister. Named party chief, Ms Sasikala also hoped to become chief minister, but the Supreme Court convicted her and sent her to jail.
Before she left, Ms Sasikala appointed her nephew TTV Dhinakaran as the party's number 2. Both were evicted last month after factions in the party led by current Chief Minister E Palaniswami and his predecessor O Panneerselvam re-united. Ms Sasikala and her nephew are fighting back.
VK Sasikala has been married for 33 years to M Natarajan, a former government public relations officer who had first introduced her to Ms Jayalalithaa. Later, Mr Natarajan was among about a dozen Sasikala relatives expelled by Ms Jayalalithaa from her Poes Garden home; she accused them of working against her interests. Mr Dhinakaran was among those thrown out.
TTV Dinakaran and S Venkatesh, both expelled in 2011, returned to the AIADMK as Ms Sasikala was headed to jail in February in what was seen as a move to help their aunt run Tamil Nadu's ruling party by proxy.
Mr Natarajan had also surfaced then, vowing to remain by his wife's side.
Ms Sasikala dropped her husband's name last year and added the prefix VK to revert to her maiden name, soon after she was appointed general secretary of the AIADMK.