Quoth Jason A. Donenfeld on Dec 08 at  4:44 am:
> Do we support them? Are we going to?
> 
> This doesn't work:
> notmuch search -tag:"This has spaces"
> 
> Nor does:
> notmuch search "-tag:This has spaces"

Tags with spaces are supported, but your shell will strip away the
quotes you've used in both examples before notmuch ever sees them.
You'll need both shell quoting and Xapian quoting in this case.  Try
  notmuch search -- '-tag:"This has spaces"'
This should be easier from a front-end, since it will take care of any
necessary shell quoting.