It may seem counter-intuitive, but you should enter your Union Dues in the Federal interview, even though it will only benefit you on your state return.

To get to the correct location to enter your union dues, in your Federal return, type "union dues" in the Search box in the upper right of your TurboTax window, then click the "Jump to" link at the top of the search results.

This solution needs to be updated for the CD version of TT Premier 2021.......the original solution is old and the steps don't work today.  Thank you. 

To enter your union dues, please follow the instructions below:

1. Open your return.
2. Click on the Federal Taxes tab.
3. Go to Deductions & Credits.
4. Click I'll Choose What to Work on.
5. Scroll down to Job-Related Expenses under Employment Expenses.
6. Click Start.
7. Answer yes for the question Do you have any employment expenses for W-2 work?
8. Follow the on-screen instructions.
9. Enter Union Dues on the page Any Other Expenses? 

@randsom