While CPI, to which LDF has allotted the seat, has fielded political debutant Bennet Abraham, BJP has brought back senior leader O Rajagopal to turn the contest into a three-horse race, as was the case in 2004.
Although it enjoys a reputation for being a progressive area, the caste and religious belief of candidates have often proven to be a key factor in deciding the outcome here.
That would explain why the CPI zeroed in on Abraham, who belongs to the numerically-strong Nadar community, to be its candidate for the seat. A doctor by vocation, Abraham is also a member of the Church of South India, to which the vast majority of the Nadar community in the area belongs.
He is also the director of a medical college run by the church.
Mr Rajagopal, and Mr Tharoor, both belong to the Nair community, which has a presence as strong as that enjoyed by Nadars in the area.
Now, if LDF's calculations work out, then things may be easy for Mr Tharoor. Last time, the CPI candidate secured 2.26 lakh votes. This, according to party strategists, is a solid Left vote bank and Abraham could get through by the addition of a good chunk of Nadar votes.
They also point out how, in 2009, BSP candidate Neelalohithadasan (now in JD-S) who, too, belonged to the Nadar community, polled 80,000 votes contesting as an Independent.
The Left would also hope that a portion of the Nair votes, and to a certain extent, the votes of the Ezhava community, will be divided between Mr Tharoor and Mr Rajagopal.
But Congress has rejected that logic. The party contends that the results of elections are not determined by caste and religion alone.
"The outcome of elections is decided by politics, issues and the stature of the candidate. All these are in favour of Tharoor," says Mohan Kumar, District Congress Committee (DCC) president.
Fighting hard to defend the seat against the backdrop of the cloud cast by his wife Sunanda Pushkar's death, Mr Tharoor has accused rivals of dragging his personal sorrow into the public domain for their narrow electoral ends.
LDF raked up the issue well before the campaign started, forcing Mr Tharoor to approach Election Commission against some LDF leaders, accusing them of having made humiliating comments in public.
A former UN Under Secretary General, Mr Tharoor had attracted the "outsider" tag when he was named the Congress candidate for the seat in 2009 with many in his party doubting his ability to cope with electoral politics. But he proved to be an astute campaigner and won by a handsome margin of about one lakh votes.
Despite controversies which have shadowed him since then, Mr Tharoor has been able to strike a chord with his constituents through his suave and affable approach.
Congress strategists say it is mere wishful thinking on part of LDF leaders that the Nadar community would vote en bloc for the CPI candidate simply because he belongs to that community.
They argue that Nadars largely are supporters of Congress, both in Kerala and the adjoining districts of Tamil Nadu. The community has no issue now with the party to decide that it will not vote for its candidate, they said.
In the 2011 Assembly polls, UDF won four of the seven segments falling under the Thiruvananthapuram Lok Sabha seat and further strengthened its hold by annexing one more seat through victory in a keenly fought bypoll.
But BJP leaders dismiss the claims of both LDF and UDF, saying there is a strong "undercurrent" in favour of the party and its Prime Ministerial candidate Narendra Modi.
That is why, BJP sources say, that party has put up the octogenarian Mr Rajagopal, a popular face acceptable to even sections outside the Sangh Parivar, as the contestant here.
Though always a loser, Mr Rajagopal has been the largest vote catcher for BJP in Kerala, finishing a close second to the Congress nominee in 2004.
"This election is basically about who forms the government at the Centre. There is a groundswell in favour of Modi, whose rally here in February was a resounding success. That is a clear indicator of the way things are turning out," asserts BJP district president Suresh.
Aam Aadmi Party is also testing the waters by fielding former IPS officer Ajith Joy as its nominee. Joy is a civil service batch mate of Arvind Kejriwal.
As in other states, the AAP is banking on the support from a large chunk of voters, especially the youth, who are "fed up with traditional politics".
Thiruvananthapuram has sent to Parliament veterans like V K Krishna Menon (1970, as a Left-backed independent), M N Govindnan Nair (CPI, 1977), K Karunakaran (Congress, 1998) and P K Vasudevan Nair (2004).
At the same time, the constituency has proven to be a slippery ground for high-profile contestants like Govindan Nair (CPI, 1980) and Jnanpith laureate O N V Kurup (CPI).
A vast, predominantly rural constituency which stretches from the shores of the Arabian sea in the west to the slopes of Western Ghats bordering Tamil Nadu, Thiruvananthapuram has a little over 12 lakh voters.
Despite the backwardness of vast swathes of the seat, the issues which prominently figure in the campaign are related to the development of the state capital and the failure to bring a high court bench here.
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