Tamil Nadu Election 2021 could become very interesting if BJP aligns with DMK before or after the election. Experts are being presumptuous if they think DMK will not ditch Congress for an alliance with BJP. Don't forget, In 1999, DMK had aligned with BJP for nearly 5 years.
2. With Congress out of power in the Centre, life is not going to be easy for a DMK led Government if they win the 2021 election. DMK is better off being neutral to the BJP rather than being strongly opposed to it
3. Given the long term situation of the Congress, Stalin can secure the future of his family aligning with a party that is likely to be in power for a long time.
Electorally however, it is unclear BJP will help the DMK much. Certainly any alliance before the 2021 election may be very risky because of the anti-incumbency against both the BJP and AIADMK may hurt the DMK should it align with BJP. The ideal situation for the DMK would be to exit the alliance with the Congress just before the 2024 Lok Sabha election. That of course assumes that DMK performs really well in the 2021 election. Exiting the Congress alliance before the 2024 election will be a good bet because with the AIADMK likely to be less united by then and with strong support from the BJP, doing well in the 2024 election would also be guaranteed.
BJP on the other hand has the best of both worlds, it has an important say in the current AIADMK Government and will continue to have a say even if AIADMK loses the 2021 election. While the current AIADMK alliance appears to be in control of the BJP, the AIADMK itself has not redeemed itself whether electorally or in Governance. That said, BJP always has an option to merge AIADMK into itself, something it is unwilling to do at this point given the risks involved.
1. Electorally, an alliance with BJP will certainly lead to loss of votes for DMK. If one takes 2019 election as base and DMK will only do better in 2021, nearly 15% of their votes came from minorities, how many of them will exit DMK-BJP alliance and how many of those will get replaced by BJP's brand strength is difficult to predict. If the BJP alliance does not replace those votes and a Congress led alliance (however small) bleeds of 5-10% of the DMK Votes will certainly hurt the DMK's chances in future elections. Also, with rapid demographic changes, what appears to be BJP's century could end as abruptly as it started.
2. BJP already controls AIADMK, the best solution for them is to merge AIADMK into the BJP rather than be in a slightly weaker alliance with DMK. It can plan much more aggressively with the AIADMK including getting its ground game in place (it has already made progress) so that eventually in 5-10 years a solid BJP party infrastructure fully replaces the AIADMK has DMK's principal rival in Tamil Nadu. Unlike other parties, the BJP has the patience and resources to wait this out.
One thing is clear when looking at all the facts, there will be no alliance between the BJP and DMK before the 2021 Tamil Nadu election. There is too much of unpredictability in it. However, how the election plays out in 2021 may decide the future course of action for both the DMK and BJP.