Flight to Lisbon

Relative to cost and duration (time), I would suggest flying on Delta to Charles DeGaulle and take a connecting flight back to Lisbon. The return flight would be through Amsterdam and then nonstop back to SeaTac.

I watch flights at MatrixITASoftware.com And I purchase flights directly through the airlines. Through the Summer, 2023, flights to Europe are expected to remain pretty expensive. My wife read today flights to Lisbon in the fall are going to be much cheaper.

How long before has become less clear post-covid, but general consensus has been that, for international travel, fares mostly go up, rarely down, and earlier is better.

Good cities to connect are the routes with good fares and schedules, which are both subjective and variable.

David... I'm not having any luck getting to the site you use...

We recently flew on Delta SEA through CDG to LIS and much preferred the route home via AMS, which is our usual choice when traveling. We flew main cabin economy. The plane to CDG was operated by Air France and had the 3 seat layout on both sides, and minimal service. We don’t expect pampering (we know we are in economy), but have found much better service with KLM or Delta. Also the flight home (KLM) was the 2 seat configuration on the sides. There are 2 of us traveling, so we prefer that seating option. We flew off season, but set the Google flight tracking and got a decent price. The flights are so full these days it can be hard to find a really good deal!

TAP flies non-stop to Lisboa from Chicago O'Hare.

TAP flies non-stop to Lisboa from Chicago O'Hare.

Even better for us left-coasters, TAP flies nonstop between Lisbon and San Francisco (SFO). It's TAP's only flight to the west coast.
It's a new-ish plane (A330-900neo).

You could hop a cheap Alaska Air milk-run flight down to SFO (look for a sale, they have them often, regular prices is about $130) and enjoy that TAP nonstop to LIS. If you do that (and book on two separate tickets) be sure to allow PLENTY of buffer between the scheduled arrival in SFO and departure for LIS. I'd want 3-4 hours buffer during summer, but Alaska flies to SFO many times a day so pick what's best for you. SFO isn't an awful airport to kill time in.

United is a partner with TAP, so in theory you could book it all on one single ticket (SEA-SFO on UA, SFO-LIS on TAP), but you'll probably pay more booking it that way.

The TAP flight from SFO-LIS looks quite reasonable to me - as low as $545 each way (random dates in July; prices vary by date, you'll want to check carefully since the flights may not go every day; check ticket details carefully, some fares don't include things like checked bag or seat selection).

Getting a nonstop from the west coast to your destination in Europe is a real treat. One worth paying a few more bucks for, IME. If I were going to Portugal this summer, I'd take a good look at this option via SFO.

Google flights: https://www.google.com/flights/

Google bought ITA Matrix.

You can set up a price alert and get notified of fare changes for particular route. You can also use the calendar feature to select cheapest days to travel.