They should go for it. Japan will kick their ass. And President Trump will become a war-time president, with nothing to lose.
Japan's military strength is low and China would overwhelm it with sheer numbers alone. If Trump keeps the presidency, then China will ultimately back down and wait out the next 4 years. If Trump loses to Harris, then it's on full steam with no U.S. intervention other than some strong words - e.g. nothing. If Harris wins, China wins.