Visually, the two words appear to differ by only a single letter s. But what differences the presence or absence of that single letter indicates! Canvas is a noun meaning strong, coarse unbleached cloth. Canvass is a verb meaning to secure someone’s vote or opinion. Although they look almost identical, canvas and canvass are two very different words.
The most common misuse of these words lies in misspelling, which no spell checker will detect. Spell checkers simply compare each word you type with a list. Every word on the list gets passed and every word not on the list gets marked. Both of these words are in that list, but the proper spelling of canvas and canvass depends on context — something spell checkers don't check.
That’s why understanding the differences here is important. Consider the cropped graphic above. Based on its function in the sentence, the word intended here is the noun, not the verb. The verb has one more s than the noun, so the more effective sentence would read like this: