I think it depends. Polls show that the ones who join the political world today after discharge, such as Tom Cotton, seem to gravitate toward the Republican Party, although Tammy Duckworth is a Democrat. During the cold War and Vietnam eras, they tended to be Democrats like John Kennedy and John Kerry, although McCain is a Republican.
As far as the common enlisted person, who knows. Some are disgruntled because the Republicans have held back on funding for the military, and in some cases, have blamed Hillary. I think they will vote for whomever they think will be pro-military.