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 was used only in The Adventures of Super Mario Bros. 3 episode "Princess Toadstool for President". After overhearing how his father couldn't act nicely enough in order to win the votes of the Mushroom People in an election against Princess Toadstool, Kooky sprayed Bowser with a dose of Nice Guy Potion, which made him act like a nice guy for a undetermined amount of time.
The potion eventually wore off and Bowser returned to being his selfish old self. Although the situation could've been remedied with another dose of Nice Guy Potion, Mario disposed the second batch prepared by Kooky and Cheatsy Koopa. With no potion forcing him to be nice, Bowser blew his chance at winning the election by accidentally scaring the Mushroom People. As a result, the only vote he received was his own.