"Vote for Hillary because it's very exciting to think of electing her. She would be such a great president."
Oh. Sorry. Just kidding. If you're looking for an argument to motivate Democrats to get out and vote, you don't want to use the one offered above, because your objective is not to get everyone a) laughing at you; or b) groaning with disgust.
As Democrats grow increasingly desperate amidst the dramatic shift in the polls toward Donald Trump, they have to offer some sort of rationale to motivate their base. It's a tough sell. Hillary is a historically awful candidate, liked by virtually no one. Even her loyalists are mere hangers-on who enjoy getting a cut of the Clintons' graft and corruption. Everyone else can see that she's a corrupt, lying sleaze.