I am a Canadian citizen without an updated passport. I want to fly from Seattle to Las Vegas. DO I REQUIRE A PASSPORT TO BOOK THE FLIGHT AND BOARD THE PLANE, OR AM I ABLE TO USE OTHER DOCUMENTATION TO SUPPORT CITIZENSHIP?

I do not believe any passports are required for travel within the US, if you were to travel on a international flight one would required.

If you are planning on entering the United States from Canada as a Canadian citizen, then you would need a valid passport to enter the U.S.

There is no passport required when travelling between states in the U.S. Although, I would be sure to bring any documentation with you that verifies your citizenship if something were to happen to you while you are in the U.S.

aircanada.com - Travel Info - Travel Documentation