Its leader, Sonia Gandhi, met Miss Jayalalitha at a well-publicised tea party last week.
The state government is prosecuting her for corruption, and Ms Jayalalitha wants to stop this.
Miss Jayalalitha took that as a sign to withdraw her support from the government, which promptly fell.
In Tamil Nadu Jayaram Jayalalitha, a former actress, wowed voters with promises of free rice and other goodies.
AIADMK's leader, Jayalalitha Jayaram, is a controversial former actress who has threatened such action before only to withdraw after winning concessions.
Chief Minister Jayalalitha announced the decision in a letter to the PM.
It has struck seat-sharing deals with two of the most awkward political partners, Mr Yadav in Bihar and Miss Jayalalitha in Tamil Nadu.
In a letter to Prime Minister Manmohan Singh, Ms Jayalalitha said people in Tamil Nadu had been angered by "barbaric acts" against Sri Lankan Tamils.
The chief minister of this state, Dr J Jayalalitha, is a big-time - albeit retired - film actress, who is as unpredictable as she is shrewd.
BBC: NEWS | South Asia | Letter: Murder, mystery and politics in India
Ms Jayalalitha - a former film star - has already set up nearly 150 all-women police stations, intended to make it easier to solve crimes against women.
And there, despite her legal handicap, the opposition leader Miss Jayalalitha has two near-constants of Indian politics in her favour, disenchantment with the state government and pre-poll alliances.
The present chief minister of the state, J Jayalalitha, was once a star actress and her mentor MG Ramachandran, who also ruled the state, was a stage actor turned movie star.
They include three women: Mamata Banerjee in West Bengal, Jayaram Jayalalitha in Tamil Nadu and Mayawati, a Dalit (the lowest caste) who ruled UP until earlier this year.
So putting a Brahmin priest in jail will not hurt Dr Jayalalitha politically - indeed, it may fetch her dividends when the state goes to the polls next year.
BBC: NEWS | South Asia | Letter: Murder, mystery and politics in India
In this election Miss Jayalalitha's lot is expected to benefit from the votes of Vanniyars, a poor caste prominent in northern Tamil Nadu whose leader deserted the national alliance shortly before the election.
Then there is the irrepressible Miss Jayalalitha, whose demands have ranged from the unreasonable (shifting the court that would try her for corruption) to the impossible (dismissing the government of her home state, Tamil Nadu).
The cardboard box from which the Jayalalitha-lookalike was doling out soft drink cans, dripped stickily onto my notebooks and elicited a quick apology -- repeated in the same vein when informing me that no, there were no bottles of potable water in lieu of the fizzy stuff.
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