You'll notice that when talking about "the stump," it is usually introduced as, "on the stump." This is an idiom in English. According to the Free Dictionary, this refers to a politician (like Donald Trump) who is travelling to different places in hopes of winning support before an election. This is because the candidate or politician used to stand and speak on a tree strump. A tree stump is the portion of the tree that is left behind after the tree is cut down. You may also hear the phrase, "I'm stumped." To be stumped means that you have a difficult time understanding something. So, you could say, "I am stumped by the meaning of 'stump' in these sentences!" :D Hope that helps!