To look at the issue from a more scientific perspective. Base on last GE result, a ragtag team from newly formed RP scored 30.67% in AMK. We can deduce that it consist overwhelmingly opposition supporters because no voters in their right mind would vote for such a team unless one is anti PAP. Hence the starting point for any opposition in AMK is at the 30% mark. What is needed is another 20% from the swing voters to make up 50%. If opposition can send in a decent team to AMK, getting above 40% shouldn't be a problem but winning is difficult because it PM ward and middleground voters will likely adopt a more cautious stance when comes to voting.
TP GRC big unknown since there is no baseline. Speaking to a few friends and people there doesn't potray the complete picture of the ground sentiment there. but they got 2 popular MP Indranee and Lily Neo. Opposition need to defeat these 2 before thinking of taking down TP GRC.
Any oppo party contesting AMK is doing the other oppo parties a favour. This is because it will force Pinky to deploy resources to AMK, as Pinky will want to gain as high a margin as possible to "prove" that his position as PM counts. The oppo parties can also claim that everyone regardless of where they are located is being given a choice to vote, even though the chance of winning may be negligible. Finally, if somehow the oppo party contesting AMK manages to gain a decent footing against the PM, it will greatly dent LHL's standing and credibility. That alone may be almost as bad a losing the seat outright and being forced to resign as PM.
BUT that being said, it would be MUCH preferred if more prominent and credible parties like WP, NSP and SFP take on AMK. If you leave it to 3rd rate or 4th rate nonsense parties like RP or SDA, all they'll do is make LHL look good.