Here, A = Trump, B = GOP congress. P(A ∩ B) is the probability of the intersection, i.e. the percentage of BOTH Trump AND GOP at same time, which CNN didn't poll as a separate field (event).
You have to understand the following
1. Since CNN poll made Trump, Rep congress, Dem congress… separate entry fields, i.e. only one field should be filled as the most important factor caused shutdown, they are mutually exclusive (cannot occur at the same time). The math only works on mutually exclusive events.
2. The rule of addition for mutually exclusive events in math is not that simple, i.e. P(A ∪ B)= P(A) + P(B), as you understood. It's P(A ∪ B) = P(A) + P(B) - P(A ∩ B). Since CNN didn't poll on P(A ∩ B, that means you won't know P(A ∪ B) , it will certainly it be less than 47%.
3. If the plus sign "+" in your "Trump + GOP Congress = 47%." isn't intended to mean the union of Trump and GOP congress (i.e. either Trump or GOP congress), instead, you meant the "+" is both Trump and GOP congress, i.e. you meant P(A ∩ B), which is the probability of the intersection, not the probability of union, then you have to apply the rule of multiplication. P(A ∩ B) = P(A) P(B|A). But CNN didn't poll P(B|A) either. So, you still don't know what you want to know. But It certainly is less than 47%.