Seeking a massive mandate, Jayalalithaa had said during the campaign that the bypoll was a result of a "political conspiracy" against her.
The ruling AIADMK had gone to the electorate on the basis of its "solid performance" of its regime, including a slew of popular welfare schemes. Almost the entire cabinet and party functionaries had sought votes for their party chief.
Though 28 candidates, including social activist "Traffic" Ramaswamy, are in the fray, the battle is between AIADMK chief Jayalalithaa and CPI nominee C Mahendran.
CPI, which is trying its luck with support of CPI(M) had sought votes to "protect the interests of working classes, the common man and democracy."
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During the campaign trial, the Left party, the only recognised political one in the fray other than AIADMK, had charged the ruling party with using money and muscle power to swing the polls in its favour.
Focussing on the "political conspiracy," the AIADMK chief while addressing a poll rally, had said that there was a "situation" wherein she could not be the Chief Minister due to a "politically motivated case." The reference was to the disproportionate assets case pursued by arch rival DMK.
Polling will start at 8 AM and end by 5 PM.
The bypoll was necessitated after AIADMK's P Vetrivel had vacated it in May, apparently to enable Jayalalithaa seek re-election.
Jayalalithaa has to fulfil a constitutional requirement of getting elected as MLA within six months of becoming CM.
She returned to the top post on May 23 after her acquittal in a wealth case. She had been convicted in the case in September last by a trial court in Bangalore.