Congress and its ally have accused the BJP of offering up to $15 million to their members to switch sides in the vote
India's opposition Congress party went to the Supreme Court for the third time in three days on Saturday in a bid to thwart Prime Minister Narendra Modi's nationalist party taking control of a key southern state.
Hours before the Bharatiya Janata Party (BJP) sought a vote of confidence in the Karnataka state assembly, Congress lawyers argued that the BJP should not be allowed to appoint a speaker.
The court turned down the plea meaning the vote would go ahead as planned on Saturday with a cliffhanger result expected in one of the most bitter Congress-BJP battles in years.
The BJP deposed Congress as the biggest party in the Karnataka election last Saturday, but its 104 seats was not enough for a majority in the 224-member assembly.
Congress saw its numbers slashed from 122 to 78 seats but it has formed a coalition with the regional Janata Dal (Secular) party which has 37.
The two parties insisted that as they have a majority they should form a government. The state governor, a BJP loyalist, chose his own party to try to form an administration, setting off the Supreme Court clashes.
A first hearing in the early hours of Thursday ruled that the BJP's chief minister candidate, BS Yeddyurappa should be allowed to take an oath of office.
But a day later it said Yeddyurappa must pass a vote of confidence to prove his majority by Saturday.
Congress and its ally have accused the BJP of offering up to $15 million to their members to switch sides in the vote.
While Modi's party has strongly denied the allegation, the Supreme Court ordered a live broadcast of the vote as "the best possible way to ensure transparency in the proceedings."
Congress and Janata Dal moved their lawmakers to luxury hotels outside the state to make sure they were not tempted to switch sides.
Reports said their mobile phones had been confiscated so they could not be contacted by rivals.
The lawmakers were bussed back to the state capital Bangalore on Saturday to take part in the vote.
Karnataka, which includes the global IT hub of Bangalore, was the last major state held by Congress.
Losing the state would undermine Congress claims to lead any political alliance against the BJP in a general election next year.