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The contest is between the BJP's Om Birla, a three-time MP and Speaker in the last Lok Sabha, and Congress's eight-term MP K Suresh. The BJP picked Mr Birla to reinforce the message of continuity.
The Speaker is elected by a simple majority, taking into account the number of MPs present and voting.
While the Opposition has 232 seats, the NDA has 293 MPs. It is also banking on the support of the four MPs of YS Jaganmohan Reddy's YSR Congress.
The stage was set for the election when on Tuesday, the Congress decided to field K Suresh just 10 minutes ahead of the noon deadline.
Its decision was spurred by the BJP brush-off to its hopes for the Pro Tem Speaker and the Deputy Speaker posts. The party had initially expected Mr Suresh - the senior most MP in Lok Sabha - to get the post of Pro Tem Speaker, which finally went to the BJP's Bhartruhari Mahtab.
On Tuesday morning, the government, while seeking consensus for Om Birla, made it clear that they are not considering a Deputy Speaker post, or the Opposition's claim to it, for now.
Congress's Rahul Gandhi said, "Rajnath Singh called Mallikarjun Kharge and asked him to extend support... the entire Opposition said we will support but convention is Deputy Speaker should be from our side. Rajnath Singh said he would call back... but he has not yet...PM is asking for cooperation but our leader is getting insulted."
Parallel meetings at both camps marked Tuesday evening. While Union Home Minister Amit Shah met the NDA allies, the Opposition Bloc had their meeting at Congress chief Mallikarjun Kharge's house.
Mamata Banerjee's Trinamool Congress has signalled that it did not approve the decision to force a contest. After publicly complaining that it was not consulted, the party came around later in the day. It has now declared support for K Suresh.
Union minister Kiren Rijiju has issued a last appeal for a consensus candidate. "For the last two days, we have been in contact with main Opposition parties, talked to their floor leaders regarding the post of Speaker... We want the Speaker to be elected unopposed and through consensus," he said.
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