According to this excellent punctuation guide:
The em dash can be used in place of a colon when you want to emphasize the conclusion of your sentence. The dash is less formal than the colon.
So, do you want to emphasize the conclusion of your sentence? The part after the colon is quite long, and it is less effective to emphasize something long than with something shorter, like this:
After months of deliberation, the jurors reached a unanimous verdict—guilty.
Either would work fine: in my opinion, there is no need to use an em-dash, for three reasons:
- the phrase just before it ("the Republican sentiment") is obviously a teaser, so we know that what follows is going to be the important part. No emphasis is required.
- the part after the colon is too long to emphasize effectively.
- this is formal writing, and the em-dash is less formal.