Rajinikanth and Kamalhasan . Both are very very popular in Tamilnadu and all other parts of South India. They are popular to some extent all over India. South Indians especially Tamils and Telugus are hero worshippers. They like whatever their hero says. keeping this point in view only Chiranjeevi has started a new party and had a bitter experience. People are intelligent But Chiranjeevi made good use of party and merged it to Congress.
Now it is the turn of Tamil Nadu. I think there also people will have become intelligent now and they can differentiate between silverscreen life and political life. So I doubt very much no one of the two will win the election if they start a new party. But if they are pulled into any of the existing parties there may be a little swing towards those parties. But it is not going to be a deciding factor. Bjp may come into power but they have given away their big man in south India to constitutional posts and I don't know who can lead that party in South India now.
I remember that Mr. Venkiah Naidu was sent to Tamilnadu when there was big problem between the to groups of AIADMK. As mentioned by Mr.Sun, if Rajinikanth goes to BJP and Kamal Haasan to DMK, the effect will be null and void. But if both of them are coming to BJP o DMk, that party will have an advantage.