Offer arrived, can I still ask for more? after 15 years as an electrician's helper

I received an offer as an electrician's helper and I think the pay is a little low. Any experience at all would help. I have a competing interview next week. What wording worked?

Thanks for writing this out. I went through something close to it 3 months ago, and it settled down once I kept a simple log of every call and form.