Easily the House, but they will have to be prepared to fight on very unfamiliar territory. The Senate is basically impossible unless they win the LA runoff or win a special election resulting from a Trump cabinet appointment.
If some conservative Democrats (who oppose the Affordable Care Act) run, Democrats have a chance to retake the Senate. There is also a slight possibility some Anti-Trump Republicans (Collins, McCain, Portman) will switch parties and become DINOs. I find retaking the House before 2022 even more difficult.