My 'leaning tower of Pisa', help please

When we moved in, there was a wooden pergola on the back of our house which was so unstable it would sway in even the lightest breeze so we quickly it took down, except that is for one upright around which the wisteria (about 16 years old and 7' tall) grew - and still grows. The problem is, that although the upright clearly needs replacing, it and the wisteria - which have become mutually dependent - both lean at an angle of about 15º off vertical.

The obvious thing is to replace the upright, but if I take out the piece of 4" x 4" the wisteria (which flowers beautifully) will fall over and I really don't want to risk losing it, however, if I leave it, inevitably at some point the whole thing will come down. We've thought about 'bracing' the wood at the bottom but a gentle push suggests it would snap at the base, apart from which that would only really be a temporary fix. Unfortunately, there's no way to support the wisteria while the wooden upright is being replaced and yet, annoyingly, if we could, there's sufficient room between it and the 'trunk' of the plant to take out the old support and drop another in.

I'd be really grateful for any suggested solutions.

 

Wisteria is quite forgiving, I replaced a whole run of fencing behind one and replaced a complete pergola supporting a under another forty-year-old wisteria, (see the photos in the "rot" thread). Just by supporting them in several places.
Without seeing it it is difficult to advise. But as has been suggested, a temporary support would work, perhaps with some soft rope attached in several places. It might be easier to saw the original post down in two or three sections starting from the top to extricate it.

 

I'd drive in a new stake before remove the old one, then strap your tree to it, just like you said. The only thing I'd be worried about is driving the new stake straight through the roots, ripping big chunks of them off.

Or, just pull out the old support, and drive a new one into the same hole perhaps. It should only take 10 minutes, and someone could hold up your tree while it is being done.

Or, you could make a temporary support like a wigwam frame while you implement the long term solution.