Is there a mileage program that will allow me to earn and redeem miles on Delta and Alaska airlines?
There is a way for the next 12 days. Alaska Airlines is not part of any alliance but it has an agreement with Delta that allows you to earn Mileage Plan miles when flying with them. Unfortunately, on April 30, 2017, the agreement will end.
Getting a Delta Skymiles membership or any other Skyteam member will get you points for flying with Delta. Alaska Airlines has its one Mileage Plan program which you should join if you want points when flying with them.
Alaska Mileage Plan and Delta SkyMiles are ending their relationship effective May 1.
As Alaska continues their relationship with OneWorld, you options to combine travel are limited.
You could go really out of the box and join Air France Flying Blue as Flying Blue continues to be a partner of both Alaska and Delta. However, you may have trouble meeting some ancillary requirements of Flying Blue such as minimum Air France flying.
This is problematic for you because Delta is increasing their presence in Seattle and Portland as a direct competitor to Alaska.
Unless Delta is somehow a specific requirement, you're probably batter off using Mileage Plan as you primary program and using Alaska partners, such as American or even AirFrance/KLM for extended coverage.