Actually, I leave it to the people to judge.
For Tampines' case, although the media talks about three parties interested in Tampines, but I think they know pretty well who is the serious player. When Mah BT made a comment, who are being approached? And who actually responded to invite Mah BT to have a policy debate. Legitimacy must be consistent both on the media front as well as the ground.
It is pretty obvious that the boxing ring is already filled with two players (actually more than that... never mind. ..) and the game has started.
If a political party is not ready to commit or willing to engage at all, then there is definitely no legitimacy to talk about. Engagement can come in various forms, most importantly, it is media responses and ground engagement.
Such legitimacy is formed in the hearts and minds of the voters, not just our own definitions. Most importantly, when more than one party steps into the ring, how would the voters view. Who are the serious players and who are the opportunists. Any political parties that value their party branding will not want to be viewed as opportunists or the "spoilers". i.e. I definitely won't want to be viewed as opportunists that just want to come and kah jiao to spoil other people's fight.
For example, I will never agree to allow any of my members to contest in Aljunied GRC or even Hougang or Potong Pasir.... unless WP or SPP agrees to let us contest while they withdraw from them. This is simple. It is not merely because of mutual respect but more importantly, I will not do something so stupid to tarnish my NSP's branding.
So, it is a matter of judgment by everyone.
Goh Meng Seng