Truthfully, Trump would suffer at the polls, as would any other Republican, if they pushed such an idea.
Well, that is obviously untrue. No republicans would abandon Trump because of court packing, infact I'm 99% sure that his approval amongst republicans would soar.
This would not play well with the broader public, including those voters in the middle who are honestly up for grabs. There would also be some Republicans who would consider such a proposal as something that could come back to haunt them should their party be out of power.