The ruling AIADMK, the seat represents the legacy of Jayalalithaa, its charismatic leader, and retaining it is vital to establish that Chief Minister E Palaniswami and his team are in control. The party has fielded AIADMK veteran E Madhusudanan, who has held the seat before and has influence among Telugu people who have a sizeable presence in RK Nagar.
Mr Madhusudanan is pitted against TTV Dhinakaran, the sidelined number 2 of the party and the nephew of Jayalalithaa's long-time companion VK Sasikala.; the pair has staked claimed to Jayalalithaa's legacy and are fighting for control of the ruling party.
Ms Jayalalithaa is seen sitting up on a bed and sipping a drink in what is believed to be the first-ever video of her in hospital. The Election Commission ordered TV channels to take the video off air, calling it a violation of the poll code.
During the campaign in RK Nagar, the AIADMK has accused team Sasikala of hiding the facts of Jayalalithaa's illness and death. There were also allegations that Jayalalithaa was dead long before the official announcement, but it was kept under wraps to give Sasikala time to gain control of the party.
Mr Dhinakaran was evicted along with Sasikala following a deal between AIADMK's factions led by Mr Palaniswami and former Chief Minister O Panneerselvam as they merged in August. Mr Dhinakaran and Ms Sasikala have challenged that decision and claim to lead the real AIADMK, though their support in the party is dwindling.
The by-election has been shadowed by allegations of bribing voters with cash and gifts. The Election Commission, however, has refused to postpone it further despite pleas from the opposition DMK.
The by-election was postponed in April this year over allegations that the faction of jailed AIADMK leader VK Sasikala paid Rs 89 crore to voters to buy support for its candidate and her nephew TTV Dhinakaran.
Making it a triangular contest, the DMK has fielded Marudhu Ganesh, a little-known lawyer but an RK Nagar local who is seen by many as a strong candidate.
While the BJP has fielded a candidate too, the Left parties and the MDMK are supporting the DMK, which wants to win RK Nagar to deliver a big psychological blow to the AIADMK and reinforce its claim that the ruling party has lost popular support after the death of Ms Jayalalithaa only months after she led her party to a big win in the assembly elections last year.