For years US military personnel favored Republicans. I was not always clear on why, I have to admit, and my stepfather was a clear exception to this rule. He fought in WW II and Korea, retired the youngest Lt.-Col. in the history of the US Army (at least up to that point) and taught at Westpoint. And was a lifelong Democrat who in his retirement took to reading The Nation. But the fact was that most US military personnel were Republicans.

That has changed starting in 2002 and as of this year deployed US military personnel have donated to Obama 6:1 over McCain.

From the above linked Daily Kos Diary:

Here are some key talking points:

1. US military 2008 presidential campaign donations, from deployed US troops, favor Barack Obama over John McCain by a 6:1 margin.

2. The Open Secrets study was comprehensive - as ABC notes, "The center tallied money from donors who list the Air Force, Army, Marines, Navy and National Guard as an employer".