All wood? Solidcore or hollow core? If it's soild core wood you might be able to strip it, wet it, and clamp it on a level/flat surface until it's dry. Hollow core, get another one.

'Un-warping' a solid wood door rarely goes very well.

You can put all the water on the door you want, when it eventually exits the wood the door will revert back to shape.

Start looking for a new door.

I have actually had good luck on Craigslist and from knockdown houses.

Offer $10 a door and the knockdown folks are usually very interested.

I agree that it's just not possible to unwarp a door. The other question is whether it's all a problem with the door warping, or whether the doorway is out of plumb as well.

"I agree that it's just not possible to unwarp a door"

I've done it with a fir door but it really wasn't worth the time, hassle, and effort. It can be done but depends on how much time you have, if you can live awhile without the door, and if you really want to put that much effort into it especially stripping it down bare.

Why did it warp to start with? The panels are supposed to slide within the "frame" to prevent this, right? If they get hung up by paint, would that cause it to warp?

"Why did it warp to start with?"

Could be anything relating to humidity or the condition of the wood or the glue when the door was constructed. During fabrication of the door components, perfectly flat wood can warp after sawing due to the release of internal stresses called "reaction forces." Sometimes these show up immediately, sometimes after the newly exposed surfaces slowly adjust their humidity content.

Any time you make a door out of wood, there is always the chance of some warpage, although yours does seem extreme. How old is this door?

"Why did it warp to start with?"

Often poor grain placement in the rails and stiles of the door (especially the rails).
The need to be relatively straight grain and quartersawn is much preferred.
See Figure 3-3 in the 'Wood Handbook' linked below for how wood changes shape based on its location in the original log.

If the grain is not parallel to the length of the board (it 'runs out') the straightness will be affected by the differential changes between tangential and radial growth rings from the tree.

No finish can stop the movement of moisture in and out of the wood, except maybe dipping it in paraffin, and that is not really a 'finish.'

Here is a link that might be useful: Wood Handbook, Chapter 3

Thanks, the door could be as much as 80 years old, but I think added later in renovation of the house. I do not think it is the best quality as it is pine with a clear finish.

"the door could be as much as 80 years old"

Doors usually warp withing a few years if they are going to.

The wood comes to equilibrium moisture with the interior of the house and assumes its final shape.

There can still be annual minor changes, but they are usually not nearly as large as the warping that should occur from bad wood in the first few years.

I agree that it's odd for it to change after all this time by so much. I'm still wondering if settling is an issue. Did you ever check whether the door frame (both sides) is plumb, or, if not, whether both sides are in parallel?

"I agree that it's odd for it to change after all this time by so much."

The question is how long ago dod it change?

Has someone simply 'lived with it' for a long time?

It is not hard to check a door for flatness.

A string tightly stretched across each face will make a reliable 'straight' line for judging flatness.

The other alternative is a long level for the height and diagonals and a shorter one for the width.

The short ones are not bad, the long levels get a little pricey if you are not going to need them again.

That door could have been like that for decades. I have only been in the house for 3 years. Thanks for your input.