That should be fine but the risk is separate bookings. If Delta is late the TAP will invite you to buy new tickets. Checked baggage would make it difficult to make the flight, you would be assuming on an early or on-time arrival for that. However note that there are regular train services from Lisbon to the centre of Porto, and that's easier just to get tickets on the day. Also perhaps consider Plus/Comfort economy seating on the direct flight, it would reduce the hassle enormously since you would enter Schengen in Porto rather than LIS or MAD.