* Congress, allies tipped to win 4 of 5 regional elections
* Graft-linked Congress ally may lose in big southern state
* Results first good news for scandal-hit gov't in months
By C.J. Kuncheria
NEW DELHI, May 13 (Reuters) - The Congress party's top ally is likely to win a key swing state in India when election results come out on Friday, giving the ruling coalition a basis to hit back at a resurgent opposition.
The maverick Mamata Banerjee's Trinamool party is tipped by exit polls to overturn 34 years of Communist rule in eastern West Bengal state, the biggest of the five states that held elections over a staggered six-week period.
Congress coalitions may also win in the southern state of Kerala and in Assam in the northeast, the polls show. The party is also expected to retain the tiny former French colony of Pondicherry in the south.
But voters in the big neighbouring state of Tamil Nadu are likely to turf out ally Dravida Munnetra Kazhagam (DMK), a party linked to the country's largest corruption scandal and which is the second biggest partner in parliament for Congress.
The overall result will be a rare dose of good news for Congress, buffeted by a raft of graft affairs and high food prices during its second term in government, and would help it regain some of its lost political authority. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ FACTBOX-State polls a test for Congress [ID:nL3E7FC0RE] A maverick battles India's communist bastion [ID:nL3E7FI126] Key political risks in India [ID:nRISKIN] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^>
With neither of the main national parties, Congress and Hindu nationalist opposition Bharatiya Janata Party (BJP), able to secure majorities in general elections, electoral power in India comes down to forging coalitions with regional allies.
The election results will take the political temperature in states that jointly make up a fifth of the 545-strong lower house of parliament and will help redraw the political map ahead of federal elections in 2014.
West Bengal sends 42 lawmakers to parliament and its long domination by the communists is one of the biggest reasons India's founding socialist ideas retain political currency even after two decades of market reforms.
But the victories will be due more to the strength of Congress's partners that give it a slim majority in parliament, underscoring the party's own weakness in the states and its dependence on fickle regional allies.
That could give the populist allies a louder voice when the government considers raising fuel prices or cutting down on subsidies -- measures that are key to keeping the fiscal deficit at the targeted 4.6 percent of GDP in 2011/12, when slowing economic growth may see a sluggish tax intake.
(Editing by Mark Heinrich)