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.