Indian police are questioning an MP in connection with the alleged buying of votes during a 2008 vote of confidence.Amar Singh is accused of offering to bribe opposition MPs to abstain from the vote. He denies the allegation.
India's Congress party-led government survived the vote over a controversial civilian nuclear deal with the US.
If the vote had been lost, India would have faced early elections. The government has been beset by corruption scandals in recent months.
Earlier this week, Delhi police arrested two people in connection with what has come to be knows as the "cash-for-votes" scandal.
Mr Singh's former aide Sanjeev Saxena was arrested on Sunday, while Sohail Hindustani, a political activist, was held on Wednesday.
Mr Singh is a member of the Rajya Sabha, the upper house of the Indian parliament.
He was the general secretary of the regional Samajwadi Party in 2008 and is accused of offering money to win over three members of the main opposition Bharatiya Janata Party (BJP) during the vote of confidence.
He has consistently denied the charge.
Earlier this month, India's Supreme Court criticised the Delhi police for carrying out a "shoddy probe" into the scandal.
In July 2008 two days of parliamentary debate on the nuclear accord ended in uproar with allegations of vote buying.
BJP members waved wads of currency notes in the air, alleging that they had been offered bribes to abstain.
India's media was awash with reports of alleged defections and desertions among MPs ahead of the vote.
The opposition claimed that a news channel had secretly recorded the alleged bribe taking. The channel handed over the tapes to parliament Speaker Somnath Chatterjee for investigation.
