Commission rate reduced over night, How should I approach this?

So one of the affiliate products that I am pushing has recently decreased their payout percentage by 5% of sale cost which is significant because it wasn't that high of a percentage to begin with. I am their top affiliate ranking #1 for all search terms related to this product. How much pull do you guys think I have to request that I be locked in at the previous rate? If I stop promoting their product they will lose a significant amount of sales. Is this poor etiquette or should I give it a try?

I'm sure they'd rather have 5% less of A LOT than 100% of NOTHING. I would approach them, and if they truly value you they will acquiesce. If they say no, then it's a marriage that is not meant to be.