• Make a large sign that has the key election dates in big dark letters
• Go to an area where people will be walking by and ideally not in too much of a hurry. Best if the area is liberal-leaning.  
• Invite people to hold the sign and take pictures of them on their own phones and encourage them to share the pictures on their social media. With their permission, also take pics on your own phone. 

(You might get more takers asking, “Can I take a picture of you with the sign?” rather than “Do you want to take a picture with the sign?”)

Next level: Ask people if they would mind if you used their pic in a slideshow video. Get their contact info so you can share it with them.