A Congress candidate may not have the unanimous support of opposition parties, sources said. Parties like Mamata Banerjee's Trinamool Congress have indicated that there is no consensus candidate yet. The Congress, which with 50 seats is the largest party, had not named a candidate to signal its willingness to work for the common goal of defeating the BJP in next year's general elections.

"It's not a blow to opposition unity despite a candidate backing out. We are together, Vandana Chavan wasn't completely decided," said senior Congress leader Anand Sharma.

The government is counting on support from the AIADMK, K Chandrashekar Rao's TRS and Naveen Patnaik, which will give it 123 votes. To win in the 245-member house, a candidate needs 123 votes, but abstention will bring down the majority mark. If the Akalis (3 seats), Sena (3 seats) and Naveen Patnaik's Biju Janata Dal (9 seats) abstain, the NDA numbers will be down to 108, when the majority mark will be at 115.

The opposition has 119 seats -- including Chandrababu Naidu's TDP and the YSR Congress. Arvind Kejriwal's AAP and Mehbooba Mufti's PDP, which between them have five seats, have indicated that they will support the opposition candidate. Support is also expected from the DMK, which has 4 seats. Altogether it gives the opposition 128 seats.

Earlier this evening, NCP leader Praful Patel said his party was reluctant to project Vandana Chavan as candidate if it was unable to get support from Shiv Sena and the BJD. Earlier, there were indications that the Sena might support the candidature of the 57-year-old former Mayor from Pune.

At a parliamentary party meeting this morning chaired by Prime Minister Narendra Modi, the BJP discussed the election. Sources said both Akalis and the Shiv Sena will support the government. The government is also counting on the support of the AIADMK and K Chandrashekar Rao's Telangana Rashtra Samithi, which will give it 125 votes.

The government's choice is Harivansh Narayan Singh, a 62-year-old first-time lawmaker from Nitish Kumar's Janata Dal United. He is described as a candidate who has friends across the political spectrum - an important qualification to manage House.

The Akali Dal, longtime partner of the BJP in Punjab, is upset by the sudden switch in candidate for the Rajya Sabha post. For a long time, their partyman Naresh Gujral was said to the front runner for the race. After the BJP's last-minute switch, the dismayed Akalis are considering the option of abstention.

The Shiv Sena is miffed that the government did not consult them before deciding on a candidate. "This morning Amit Shah called Uddhav Thackeray and asked for support. We will take a call an hour before the voting," said senior party leader Sanjay Raut. The party, he said, will take into consideration whether a capable person has been fielded.