to visit (e.g. voters) in order to solicit political support or to ascertain opinions
to discuss or debate (something)
to examine (something) in detail
NAmer to examine (votes) officially for authenticity
to solicit orders, votes, opinions, etc
canvasser noun
obsolete canvas to toss in a canvas sheet, from canvas1. Tossing in a sheet was a rough affair, often done as a punishment; the sense evolved via ‘to handle roughly’, ‘to castigate or criticize’, ‘to debate vigorously’, ‘to actively seek support’. See also usage note at canvas1