Congress, which lost control of the national government to Modi in 2014 is desperate to cling on to Karnataka
India's opposition Congress party suffered a major electoral blow in its last major state on Tuesday and was scrambling to build a coalition to stop Prime Minister Narendra Modi's party taking over.
Modi's Bharatiya Janata Party (BJP) led the count in Karnataka but fell short of a clear majority in the state of 60 million people, which includes the wealthy global IT hub of Bangalore.
The result is being closely watched before a national election next year.
The BJP had won 87 seats by Tuesday afternoon and was leading in 17 others, according to figures from the Election Commission of India.
Modi's party needs 113 seats to secure a majority and complete the humiliation of Congress, which was tipped to win 78 seats, a huge fall from 122 in the previous assembly.
Congress, led by Rahul Gandhi, announced it would offer "unconditional support" to the third-placed regional party Janata Dal (Secular) to form a government.
"That's the best way to keep the BJP out of power," incumbent Congress chief minister Siddaramaiah told reporters in Bangalore.
A Congress-Janata Dal alliance should have a majority in the 224-member assembly.
The BJP accused Congress of betraying voters to salvage its "pathetic defeat."
"Congress, despite having been rejected by the people, is trying to gain a backdoor entry," BS Yeddyurappa, the BJP's chief ministerial candidate for Karnataka, told reporters.
A record 70% of the nearly 50 million voters turned out in Saturday's election.
Congress, which lost control of the national government to Modi in 2014 is desperate to cling on to Karnataka.
Without it the party would control only three states - Punjab, Puducherry and Mizoram - which together account for just 2.5% of India's 1.25 billion people.
The BJP rules 21 out of India's 29 states. Since its landslide national win in 2014 the Modi juggernaut has stormed northern India, claiming around a dozen crushing state victories.
Modi and Gandhi traded barbs at rallies before tens of thousands in the Karnataka campaign.
Gandhi, the scion of India's Nehru-Gandhi dynasty, had hoped Karnataka would be his first election victory since taking over as Congress leader from his mother Sonia this year.