Do past refusals or a US visa rejection hurt my new Canadian application?

A past refusal — Canadian or American — is not an automatic bar, and officers see reapplications succeed all the time. What matters is that you declare every refusal honestly when the form asks, and that your new application answers the reasons for the old refusal. Hiding a refusal is misrepresentation, which can bring a five-year ban and is far worse than the refusal itself. A stronger, honest file that fixes the earlier weakness generally gives you a real chance.