A SP-Cong tieup in UP will push BJP third place, probably not opening its account - losing 15 seats they hold today.
A SP-JDS-Cong tieup in Karnataka commanding 55% vote share will ensure BJP will not open its account - losing 18 seats they now hold.
Orissa, MP, Rajasthan & Chattisgarh - NDA/BJP governments face strong anti-incumbency -BJP expected to significantly lose seats
WB - a Congress-TMC tieup will imply Congress/UPA more seats at the cost of Commies. Kerala the Commies are destined to a white wash. Cong to gain 20 seats
And this is the short list.
So tell me where does BJP or for that fact the Commies or BSP expect to gain to offset these losses? That's why they contemplated that they will cast aside their ideological differences and roping the SP they could topple the UPA.
Unfortunately they failed and ended up with mud on their faces.