Schumer and Pelosi don't give a damn about the border or not. It is just a power game to them. It could have been anything else that POTUS believes in. To Schumer and co. the issue is about control and dominance. They want to control the agenda by being obstructive and calling the shots.
Don't get sucked into believing anything Schumer/Pelosi say about what they believe or want. The only thing they want is control to implement their longer term goals. The 16 year plan is on ice, but they will still want to continue the moment they have an ounce of power or ability. One straw at at time.
Here is the rub though. POTUS is no ordinary politician. He understands the dynamics of the power game and is certainly a grand master at it. Schumer and Pelosi are good at the game, but they have the mistaken idea that POTUS cares what people think about him (since popularity is the bread and butter of normal politicians). They are projecting their own insecurities on POTUS.
They will lose this time and every time they try to grandstand.