Political crusaders have long sought to lift up the downtrodden, comfort the afflicted, and empower the defenseless. Scott Crider, the founder of Dogs Against Romney, says he fits squarely into this grand tradition. His mission: make sure every American voter knows that once, 29 years ago, Mitt Romney, drove for 12 hours to Canada, with the family’s Irish setter, Seamus, in a crate on the roof. “I’m just a guy who is trying to give a voice to the voiceless,’’ said Crider, a 47-year-old from Gulf Shores, Alabama.