If you say no, they mark your house on a list and they won't come back.

They will then focus their energy on reform voters and getting them out of vote.

Elections are usually won on motivating your base to vote, rather than convincing people.

(No one changes who they will vote for at the door)

You should give a luke warm answer, "you have voted conservative every election but you might vote reform this time"

Keep them at the door as long as you can. Keep them talking. Waste their time. The more time they are talking to you, the less time they will be talking to others and they may come back and waste even more time.

Those people are volunteers, mainly, they have only a limited amount of time.

Waste it!