This is not Trump Russia related, but has context to current and potential Trump favorability with a subset of his base:
All branches of military, especially those in the top ranks, favor the republican party because it is the party that has historically buttered the military up with spending and high profile inclusion in political policy. So for the Marine Commandant to openly speak out about a political policy, let alone a republican one, that will have an unforeseen impact.
A lot of your conservative radicalism happens in the US military. Military personnel will share the internal sentiment about policy and leaders of the country with their families back home in the low-middle to lower class regions. These families will embrace and regurgitate these sentiments because of the stature their family members hold in the community (i.e. being a military man or woman). We already know how susceptible they are to propaganda and their general lack of understanding of politics, so these family members tend to be a bigger influence on them than Fox or Trump's twitter feed.
I say all of that to say, the longer Trump is in office, the more he will unintentionally bleed his own base. We're actively seeing it now.