In total Eastern part of India accounts for 127 seats from West Bengal, Odisha, Jharkhand, and Bihar.

In these BJP and its allies had strong presence till now only in Bihar and Jharkhand and marginal presence in other states. But in this election trends indicate it might turnaround for BJP, due to strong aggressive campaign and foundation being put by BJP since last 4 years.

Let us look at the trends in these 4 states in detail.

Bihar
Total Parliamentary seats - 40

From 2014 there has been so many changes in the Bihar political landscape and also again there is a chance some more re-alignment might happen before the 2019 elections.

In 2014, there was NDA (with BJP, Ram vilas Paswan, and Kushva) which won the most number of seats (30), followed by RJD + Congress alliance and Nithish kumar's JDU  contested seperately won only 2 seats.

Then in 2015 assembly elections there was Maha Gatbandhan comprising RJD + JDU + Congress that won a landslide victory. But then in 2017, Nitish kumar walked over from MGB back to his old ally NDA, and the situation has now again turned. There might be impact on the overall seats if other strong ally Ram Vilas Paswan who has pockets of influence in certain areas walkover to MGB - then in that case the seat tally might reverse to what we predict today.

So based on the current alliance the indicative seat tally is,
Total Seats - 40
NDA (BJP+ JDU + LJP + Kushva) -  27 to 30 seats
MGB (RJD + Congress, CPI(M) + NCP) - 10 to 12 seats

Note: if the LJP and Kushva walk over to MGB, these numbers will probably reverse :)

Jharkhand
Total Parliamentary seats - 14

In the last Parliamentary election, BJP won the maximum number of seats. Then in 2014 assembly elections - BJP won a majority and there has been a stable government first time in Jharkhand for last 4 years. However these seems to be some anti-incumbency against BJP, and that might reduce the BJP seat tally

Indicative Seat tally
Total Seats - 14
BJP - 8 to 10
UPA (JMM + Congress) - 6 to 8
JVM - 2 to 3


West Bengal
Total Parliamentary seats - 42

In 2014 elections there was clean sweep for Mamta didi's TMC winning 34 seats, and rest of the parties like Congress, Left front and BJP winning 2 to 4 seats. This time around BJP has been aggressively campaigning in the state and seems to be projecting itself as a alternative to TMC, with Congress and Left front base getting significantly weakened over years. Some of the pollsters are project in upwards of 15 seats for BJP, but I feel it will be a too high number. Still whatever gains BJP will make here, they will be looking at primarily to offset the losses they will be suffering in West and North where they had peaked in 2014 elections.

My projection is below.

Indicative seat tally
Total number of seats - 42
TMC - 30 to 32 seats
BJP - 6 to 10 seats
Congress - 2 to 4 seats
Left Front - 2 seats

Odisha
Total Parliamentary seats - 21

In 2014 elections, despite the Modi wave throughout the country, Naveen Patnaik's BJP had swept the election with 20 seats. This time around, there seems to be anti-incumbency catching up with BJD. Odisha also goes to assembly elections in 2019 along with General elections, and BJD has been in power here from 2000. So along with anti-incumbency, aggressive campaign by BJP seems to be catching up with BJD. In the 2017 Panchayat elections, BJP performed impressively and that seems to be an indication for upwards trend of BJP. Congress has now reduced to marginal leaders with most of its well known leaders joining either BJD or BJP. So seems like this time around it will be direct contest between BJD and BJP, with congress still influential only in few pockets.

My projection is below

Total seats - 21
BJD - 10 to 12
BJP - 6 to 10
Congress - 0 to 2

Summary
BJP would like to retain their current seats from East (mainly from Bihar, Jharkhand) and make significant gains in West Bengal, Odisha to offset its losses in West and North regions. Trends seems to indicate that BJP is progressing in that path very well

For other other national party, Congress there is not much hope in West Bengal and Odisha. They will be banking on their allies to make gains in Bihar, Jharkhand to improve their tally.