South has not been strong hold of BJP and used to be strong hold of Congress till 2014. Now after bifurcation of Andhra Pradesh in 2014, and no mass leader for Congress in Andhra (after passing away of YS Rajasekhara Reddy), even in Andhra Pradesh and Telangana, Congress has become marginal player.

In total these 5 states account for  129 seats.

Let us look at these 5 States in South and see where NDA, UPA and others stand per the current trends.

Karnataka
Number of Parliament Seats: 28
Recently in May 2018, there was a Assembly elections. Due to spirited campaign by Narendra Modi - BJP came back from behind to finish as the leading party with 104 seats, just stopping 8 short of majority. This resulted in unholy alliance of Congress + JDS to combine to form the power with sole aim of stopping BJP from forming the government.

Now for the Parliamentary elections - Congress + JDS are talking about alliance, although seat sharing discussions is still in progress. JDS is strong only in certain pockets, hence this alliance will have only marginal impact on the final seat tally. JDS has been mostly winning elections in Mandya, Hassan and Bangalore Rural constituency. They has some presence in Bangalore North, Mysore-Kodagu, Udupi, Shimoga, Chikkbalapur constituency.

Brand Modi is quite big in Karnataka and add to this solid RSS organization behind the BJP, BJP will definitely again end up leading the seat tally like it has been doing since 1998 elections

Indicative Seat Tally
Total Seats - 28
BJP - 12 to 16
Congress (in alliance with JDS) - 7 to 10
JDS (with / without alliance with Congress) - 3

Kerala
Number of Parliament Seats - 20

Main fight is between UDF (lead by Congress) and LDF (led by CPI(M), BJP led NDA is very marginal player here. Riding the Modi Wave in 2014, they gave a very close fight and lost the Thiruvananthapuram seat (retained by Shashi Tharoor) only by 10,000 votes.

This time around too BJP / NDA is going to be a marginal player - not much chance in any seat. Unless some body like Mohan Lal enters the fray for BJP. But again movie stars do not make much impact in Kerala.

Like last time this time around seems like UDF will win higher seat share.

Indicative Seat Tally
Total Seats - 20
UDF - 12 to16
LDF - 4 to 8

Tamil Nadu
Number of Parliament Seats - 39

There are lot of changes in Tamil Nadu since the last election, hence it seems like for pollsters it will be difficult to predict the scenario in Tamil Nadu.
Two political heavy weights Jayalalitha and Karunanidhi have passed over last couple of years - hence AIADMK and DMK are now both lead by different leaders. DMK is more better in this case because MK Stalin is a more know face compared to the people at the helm in AIADMK.  Then there is one more faction of AIADMK lead by V.Sasikala group. Two movie stars Rajanikanth and Kama Hassan have also thrown their hat into the ring. But politically they are novice - do not seem to have made much political impact so far.

So overall all political parties are starting from ground zero in this election, with DMK holding better advantage due to presence of MK Stalin. DMK will be going along with Congress. AIADMK might align with BJP.

Last time around AIADMK swept the state (with Jayalalitha at the helm), and BJP riding on Modi wave could get only one seat. This time around Modi wave is not there in Tamil Nadu, so do not expect AIADMK to benefit much from this alliance

At this stage DMK stand to make major gains if not sweep basted on the current trends.

Indicative Seat Tally
Total Seats - 39
DMK + Congress - 25 to 30
AIADM + BJP - 5 to 9
Others - 2 to 5

Telangana
Number of Parliament Seats - 17

Telangana last time had first election after the bifurcation, and this will be second election

TRS current ruling party in Telangana has advanced the assembly election which is going to happen in November, the result of that will definitely have a bearing on the Parliament election next year.
In last election the main contenders were TRS, Congress and TDP + BJP fought as alliance. This time TDP has broken away from BJP and going with congress and this will have impact on the overall seat tally. If the TRS decides to ally with BJP, then it will improve both TRS and BJP seat tally else - there will be reduction in TRS seat tally.

Indicative Seat Tally
Total Seats - 17
TRS - 8 to 10
TDP + Congress - 5 to 7
BJP - 0 to 2

Andhra Pradesh
Number of Parliament Seats - 25

Currently ruled by TDP with main rival being Jagan Reddy led YSRC.
Last time TDP had gone into the election in alliance with BJP, and this time they seem to be going along with Congress. If there is no alliance, then seems like YSRC will sweep the election.

Indicative Seat Tally
YSRC - 11 to 15
TDP + Congress - 7 to 10
BJP - 0 to 2

Summary
Overall South again is not looking promising for BJP,  they may not be able to make much gains here to offset the possible losses in North and West regions. They should look at improving their tally in Karnataka and hope to get into alliance with TRS for possible gains in Telangana to offset the losses in Andhra Pradesh

Congress on other hand expected to well and improve on their last tally due to their alliances in all 5 states. Need to how much this improved seat tally will help in Congress led UPA overall tally.