Air to Portugal

In a quick search, I see that United and Air Portugal both have nonstop flights from Dulles, so one of those would be my first choice, if fares aren't out of line. British Airways has a one stop that connects at London Heathrow (airport taxes may be high, but you're closer to the continent), or they code-share with American departures from other DC-area airports that require a connection in the US, such as Philadelphia or Dallas. I guess those would be my second and third choices, if fares are otherwise similar. Perhaps you have preferred airports, yourself.

American Airlines has a seasonal flight thru Philadelphia into Lisbon.
Delta flies thru JFK, and a flight from Boston starts 5/24/2019.
United flies to Lisbon out of Newark. But they do have a seasonal flight from Dulles--whatever season means.
TAP Air Portugal starts flying to Lisbon from Dulles 6/16/2019.

I flew United nonstop in September. Good flight

wright,

If you don't mind one stopover on the way to Lisbon, you could also look at flights from Toronto to Lisbon, using Air Canada Rouge. Rouge is the budget branch of Air Canada. If you ever fly with them (especially for a long flight), I'd highly recommend paying extra for Premium Economy as the regular economy seats are somewhat "spartan".

That flight (AC-1916) has a travel time of about 6H:45M, and at the time I'm posting this, the flight is in the air and about half way to Lisbon.