These two spellings are often confused, but they mean quite different things.
Canvas means a tough material and it was often used to make tents and to cover deck chairs, as in:
During the Second World War, soldiers were often housed in tents made from canvas.
The verb canvass means to campaign, to electioneer, to drum up support or to solicit votes, as in:
The politician visited most of the homes in his electorate as he tried to canvass support for his re-election.