Jeremy Teigen does a guest post at the Monkey Cage examining the impact of past military service on electoral success in House races from 2000–2006:
In general, veteran status has small effects that are not statistically distinguishable from 0. Democratic vets did better than their nonveteran peers in 2002, but did no better in 2006. That election was the year that Joe Sestak, Tammy Duckworth, and others constituted the “Fighting Dems,” a year when you would expect Democratic vets to do well, but instead Republican veterans were helped by a martial past. While the Fighting Dems may have helped the Democratic tide in 2006 by helping Democrats credibly criticize the Bush administration’s conduct of OIF (and OEF), Democratic veterans actually did a little bit worse than Democrats without a service record. This result may have occurred because Democrats were overzealous in their attempts to attract veterans as candidates, leading them to select veterans over higher quality challengers (14% of Democratic challengers were vets in 2002, compared to 28% in 2006). Republican vets running that year performed a little better than nonveteran Republican candidates, as they had been doing in the previous three elections, but the advantage just slipped above statistical significance. Overall, the effect of veteran status is very small.
Basically, there’s no convincing evidence that recruiting veterans is a good strategy, but it’s close enough that people who intuitively feel it’s a good strategy will be able to keep feeling that way.
It does seem to me, though, that as a governance matter it’s useful to have veterans in the congress. In general, congressional oversight of national security issues has tended to be lackluster. To improve things I think it’s important to have members of congress who feel personally comfortable discussing all aspects of this stuff. That means veterans. But it also means people with Foreign Service backgrounds and people with intelligence backgrounds and all the rest.