Selective prosecution to try to sway an election was rejected once by voters after when Bill Clinton was impeached for his affair. So this will not directly boost Democrat's chances, might even make Donald a martyr. But would make sense if the strategy was to split the vote on the right.
Trump being in legal trouble is the carrot that got DeSantis to run. The problem is if DeSanits defeats Trump in the primary, he can't win in the general precisely because he defeated Trump. Trump has too many supporters that will never switch sides.
