What is required for me to sign a joint return for my spouse who has dementia? I have a durable power of attorney for financial matters.

If you have durable POA, you can fill out form 2848 and attach the durable POA with the form and return. 

You can sign your name and POA for her signature block. 

Thanks for your prompt reply.  In this case I am E-filing the federal return via Turbo Tax.  I found a related response by ToddL 99, a Turbo Tax Expert Alumni dated 3/16/2021.  Todd advised in such instance to prepare Form 8453 and 2848 and submit to the IRS within 3 days of acceptance of the return, and to mail them to Austin TX.  I wss planning on attaching my durable power of attorney as authority to sign the Form 2848.

As to the state return, i plan on following your advise of preparing  the Form 2848 for mailing with the return along with the durable POA.

Does the above approach appear acceptable?

Yes.  That sounds like it should be fine.  The most important thing is proving that you have the authority to sign and at this point the POA is the evidence needed to complete the rest of the tasks as stated.