McCaskill points out the elite vs. grunts difference and these other points also mimic the general population:

Military men appear to be more supportive of Trump compared to military women. Among men, the survey shows a 43 percent favorable view, while among women service members, 53 percent of women expressed a “very unfavorable” rating of Trump and 56 percent responded negatively.

The survey also shows a gap among white and non-white service members. Among whites, 46 percent of troops had a favorable view of the president, versus 45 percent with an unfavorable view. Yet among non-white service members nearly two-thirds responded with a negative view of him.

BUT the change illustrated in this graph is extremely striking:

And note the writer's analysis as regards that:

Some of the shift in military sentiments could be linked to the firing of Mattis, who a year after his dismissal still enjoys an exceptionally high — 86 percent — favorability rating among all service members in the poll.

Trump’s replacement for Mattis, current Defense Secretary Mark Esper, does not inspire strong feelings one way or the other. Esper drew a 24 percent approval rating from troops and a 20 percent disapproval rating, with 56 percent saying they have no strong opinion of the Pentagon leader.

Mattis' resignation was reported Dec. 20, 2018