Who gets to participate in the Canadian political debates?

There is a lot of discussion right now in Canada around the fact that Elizabeth May, leader of the federal green party, has not been invited to the national debates again. This happened initially last time as well, but that decision was eventually rescinded due to public backlash. It's likely to happen again based on what I've seen. But it makes me wonder a bigger question, what criteria should we be using to determine who gets to attend the national debates?

 

 
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