Transaction isolation and UNION queries

I have a question about transaction isolation that I can't find an
answer to in the docs. I'm working with a database that has some data
split over two tables. One table is the ultimate destination for all
the data, the other is a "pending" table which holds rows during data
entry. Rows from the pending table are moved to the permanent table
once data entry is complete. For some purposes I want to see rows from
both tables, so I do a UNION. My question is, with only read committed
isolation, could a commit by another transaction make changes appear
between the separate parts of the UNION query? In other words, could a
row appear to be missing or duplicated because a transaction that was
moving the row from pending to permanent committed while the UNION was
running?

Thanks!

Bob Smith
Hammett & Edison, Inc.
bsmith(at)h-e(dot)com