Tamil Nadu and Kerala are voting in single-phase for 234 and 140 assembly constituencies respectively.
In TN, the ruling All India Anna Dravida Munnetra Kazhagam is contesting the poll in alliance with the Bharatiya Janata Party.
While M K Stalin's Dravida Munnetra Kazhagam has joined hands with the Congress.
Actor-turned-politician Kamal Hassan's party Makkal Neethi Maiam and T T V Dinakaran's Amma Makkal Munnetra Kazhagam (AMMK) are also in the fray.
In Kerala, Congress-led United Democratic Front is seeking to oust Chief Minister Pinarayi Vijayan's Left Democratic Front government, while BJP-led National Democratic Alliance is the third constituent in the triangular contest.
The presence of the BJP, which is positioning itself as a political alternative with candidates like the 'Metroman' E Sreedharan will impact the attempts made by both the LDF and the UDF to swing the constituencies to their favour.
Rediff Labs analysed the 2016 assembly election results to develop a Sentiment Meter for the 2021 battle in Tamil Nadu and Kerala.
A mathematical model is used to calculate how seat distribution will be altered as sentiment/percentage of votes towards a contesting political party changes.
Based on the change, each constituency is marked with the winning party.
A swing represents that the victory margin is close to the runner-up party.
Move the pointer on the Sentiment Meter (depending on who you think will get more votes) towards the political parties to see how the outcome is likely to change.