Is strongly correct in the following, or should it be strong?
… and had a strongly Protestant and unionist identity.
What is the explanation in grammar terms?
Both are correct, but strong is a bit better.
Strong Protestant and unionist identity says that the 36th Division had a Protestant and unionist identity, and that sense of collective identity was intense.
Strongly Protestant and unionist identity says that the 36th Division had a sense of identity, and that identity was intensely Protestant and unionist in character.
The difference is subtle. I think both statements are accurate, but the former is closer to what the article means to say. So strong is the right word to use.
Martha says that strong Protestant and unionist identity is strictly speaking incorrect unless you add commas. I disagree. It’s completely standard. It is easy to find plenty of examples of this pattern in good journalism and academic writing. Here are some examples (found in the Corpus of Contemporary American English):
strong economic and political ties
a strong moral and ethical impulse
an Iranian culture with strong Hellenistic and Indian traditions
complex social and economic conditions
overall physical and mental health
alternating hot and cold water
There are thousands more.
The pattern also appears in the name of the National Oceanic and Atmospheric Administration, a U.S. government agency.