Nice Guy Potion
From the Super Mario Wiki
Nice Guy Potion is a concoction created by Kooky von Koopa, which forces anyone who inhales it to act and behave like a kind, sensitive person, even if the being it's used on is naturally mean and spiteful.
Nice Guy Potion is used only in The Adventures of Super Mario Bros. 3 episode "Princess Toadstool for President". After overhearing how his father could not act nicely enough in order to win the votes of the Mushroom People in an election against Princess Toadstool, Kooky sprays Bowser with a dose of Nice Guy Potion, which makes him act like a nice guy for a undetermined amount of time.
The potion eventually wears off and Bowser returns to being his selfish old self. Kooky and Cheatsy Koopa begin to prepare another batch of Nice Guy Potion, though Mario disposes of it. With no potion forcing him to be nice, Bowser blows his chance at winning the election by accidentally scaring the Mushroom People. As a result, the only vote he receives is his own.