What political obstacles would need to be overcome for Canada to consider joining the EU?

Canada and the EU are close trade partners and seem to be natural allies on the world stage (https://www.eeas.europa.eu/canada/european-union-and-canada_en?s=220). Given that the Trump administration has made both economic and territorial threats against both Canada and Greenland, I am wondering what sort of political obstacles might need to be overcome to facilitate entry of a non-European country into the EU.