I've canvassed many times, and, while there is usually a script it is mostly "Hello, I'm ____ with the ____ (camapign, organization, etc)and I'd like to ask if we can count on your vote this election"
As in, it's as much a survey and a get out the vote. Not a, here are the arguments why you should vote for this candidate and/or not vote for another candidate. If the person seems open to talking about the candidates, then by all means talk about the candidate. If not, don't push it.
Never have I encountered, been asked to speak badly about another Democrat (or even about the Republican).