Graham Platner’s seat would not be filled by a single automatic successor on the ballot; Maine Democrats would likely have to choose a replacement nominee quickly if he drops out. The names most often being discussed are local and state-level Democrats, but no official replacement has been selected yet.

What the reporting says

Maine’s ballot-deadline rules mean Platner could still be replaced if he withdraws soon, but the party would then face a short scramble to pick someone new. Reporting says that uncertainty is centered on a very small window of time, not a settled heir apparent.

Who is being mentioned

Articles discussing possible replacements point to several potential contenders rather than one clear front-runner. Those lists are speculative, and they reflect who could credibly step in, not who has actually been chosen.

Practical answer

So the direct answer is: no one yet. If Platner exits, Maine Democrats will likely nominate a substitute candidate in a rushed internal process, and that person would then become the party’s Senate nominee. TL;DR: Platner does not have a designated replacement at this point, and the party would have to pick one quickly if he leaves the race.