Actually it might pull down WP votes further.
Many sinkie especially the hardcore opp core like you and moi ish fail to see how most sinkie especially the so-called middle ground fear the SDP due to decade of bad leeputations and their rightist approach in the past.
WP managed to gain the ground today ish because they have managed to stay under PAP and MSM radar long enough and manage to move away from the rightist image from JBJ times until their surprise emergence to buttsack the higher mortals in 2011. But after that the negative MSM attention also start to swift on to them and we can see some of the result today liao. This same kind of negative MSM attention in the past has been targeting the SDP for decade, so you can easily judge the damage done when even WP also can't spare and lose a few percentages.
If they merge, the negativeness will also merge together and more sinkie will siam away from WP than they can gain from the merge.
Think SDP need a leadership change like how WP leadership ish transfer to LTK in the past to show the middle-ground sinkie that they are a new non-rightist party plus at least another decade of under radar hardwork on the ground before they can win anything.
