Parties and States, big and small, have been pledging support to the GST Bill. So why was the BJP still trying to convince Congress to vote in favour of it? Here is why:

The Centre is keen on passing the Goods and Services Bill in the current session of Parliament. The Bill, officially known as The Constitution (122nd Amendment) (GST) Bill, 2014, requires amending the Constitution to bring a goods and service tax that will subsume all indirect taxes.

Constitution Amendment Bills can be passed only if at least two-thirds of the House votes in favour of the Bill. Both Houses need to pass the Bill separately in this manner and a joint session is not allowed. Then the Bill should be ratified by at least half of the States.

The Narendra Modi-led government is the first one since 1984 to enjoy a single-party simple majority in Lok Sabha. The BJP has 282 MPs, well above the half-way mark of 273. However, even with allies, its strength in the Lower House is 336, short of the two-third majority mark of 409.

The Bill cleared the Lok Sabha with the support of regional parties such as the Trinamool Congress.

But the government is up against a big hurdle in the 245-member Rajya Sabha. In the recently-concluded Rajya Sabha elections for seven States, the BJP managed 11 out of 27 seats. This took BJP's strength in the Upper House to 54. Other constituents of the NDA collectively hold 18 seats taking the alliance number to 72. The two-third mark is 164.

The Rajya Sabha has 10 nominated members, of whom seven were nominated this year. Out of four independents, media baron Subhash Chandra, and industrialists Rajeev Chandrashekar and Parimal Nathwani won with the BJP's support. Assuming that these 10 MPs vote for the government, the support of 82 members is still needed.

The JD(U), RJD, INLD and JD(S), which have considered a merger to bring back the parent Janata Parivar, together hold 15 seats. The JD(U) and RJD, the ruling combine in Bihar, account for 13 of those seats. It may be noted that friends-turned-foes Nitish Kumar and Lalu Prasad turned friends once again during the Bihar Assembly elections to defeat BJP in the State. However, bringing some respite to the BJP, Bihar Chief Minister Nitish Kumar has announced that his party's 10 MPs will support the Bill in Rajya Sabha.

Perhaps for the first time, 77 members in the Rajya Sabha are not aligned to the principal national parties, BJP and Congress. The SP (19), AIADMK (13), Trinamool Congress (13), BJD (8) and TRS (3) are all ruling parties in their respective States. The SP and BSP (6) have pledged their support. The Sharad Pawar-led NCP has five members, and JMM and YSRC have one each. The government hopes these parties will give issue-based support. While the BJD has sought some changes to the Bill, the AIADMK has outright rejected it.

The Left parties, which have nine members, have already announced that they will oppose the Bill.

The Congress, which didn't have enough numbers to hold the post of Leader of Opposition, is the single largest party in Rajya Sabha with 60 members. Its UPA constituents -- DMK, Muslim League and KC(M) -- hold six seats.

If the BJP manages to convince Congress, then it gets the required number. Though the idea of GST was floated by Atal Bihari Vajpayee, it was P. Chidambaram, the UPA-1 Finance Minister, who announced that GST would be rolled out in 2010. An Empowered Committee of State Finance Ministers was constituted during UPA regime and the Bill, in current form, is based on this Empowered Committee's report. But Congress is opposing the Bill in the current form and has three demands.

The BJP has done away with the one-per-cent additional levy on inter-State tax but is unwilling to accept Congress' other demand: constitutional cap on the rate of GST up to the maximum of 18 per cent.

Another reason: BJP is the ruling party in nine out of 31 States. While Congress is the ruling party in seven States and coalition partner in two.Together, they can pass the Bill in their respective States, which will enable quicker implementation of the Bill.