I'm sure there are many reasons why President Obama nominated Chuck Hagel to be secretary of Defense, but the fact that Hagel is a Republican surely played at least some part. After all, if he nominated a Democrat to head the Pentagon, congressional Republicans would surely oppose the nomination and charge that the nominee was too dovish. Which of course is exactly what has happened with Hagel (along with some truly despicable phony accusations of anti-Semitism*). I'm not the first liberal to be disappointed with the fact that Democratic presidents seem to feel the need to placate their opponents by picking Republicans for this particular position. As Michael Beschloss observed, Republican presidents have never picked a Democrat for this job, but about half the secretaries of Defense in Democratic administrations have been Republicans.
What's most important to note about this is that there is no equivalent on the other side. Republican presidents don't feel the need to appoint Democrats to lead agencies whose missions are traditionally associated with Democrats. In fact, they have no problem appointing people whose goal is to fight against the missions of the agencies they lead. For instance, the role of the Labor Department is to enforce labor laws and protect workers. So do Republican presidents say, "This department's mission is kind of liberal, so we'll just appoint a Democrat to be Secretary"? Hell no. Look at recent Republican labor secretaries. For George W. Bush's entire tenure, the office was held by Elaine Chao, who had been a fellow at the Heritage Foundation, as well as an official in prior Republican administrations. Today she's back at Heritage and serves on many corporate boards. She's also married to one Mitch McConnell, whom you might have heard of. Needless to say, she didn't get too many awards for service to America's working men and women. The job was held in the George H.W. Bush administration by Elizabeth Dole, and after her by Lynn Martin, who had been a Republican member of Congress. Ronald Reagan appointed two corporate executives and a Republican senator during his time, including Ann Gorsuch, a hard-right conservative politician who spent her two years in the job working like a beaver to destroy the agency and everything it stood for. You get the idea.
I'm not saying the two are parallel, since the Republican approach to agencies they don't like, such as Labor, Interior (remember James Watt? Gale Norton?), or the EPA has been to appoint people up and down the ladder who will work to undermine the agency's mission, while Democrats have never done anything of the sort with the Pentagon, nor am I saying they should. And I don't have a problem with Obama appointing Republicans to his cabinet, if he finds what he believes to be the right individuals (the other Republican in the cabinet, Transportation Secretary Ray LaHood, seems to have done a fine job). But maybe Democratic presidents should stop acting as though they're afraid someone will call them wimps if they don't pick Republicans to head the Pentagon.
* Now, about those anti-Semitism charges. For a taste of what they're saying, you can read this odious column in the Wall Street Journal, the gist of which is that Chuck Hagel is an anti-Semite because he once referred to the "Jewish lobby" intimidating members of Congress. Now first of all, the term "Jewish lobby," inaccurate though it may be to refer to AIPAC, is often carelessly tossed about, including by many of AIPAC's allies. Furthermore, as I've argued before, even the more common "Israel lobby" is a misnomer, because what AIPAC lobbies for is not Israel per se, but a particular ideological faction within Israel, which is why they would more properly be called the "Likud lobby." Second of all, of course AIPAC intimidates members of Congress; that's the business they're in, and they do it very successfully. And finally, if there's anyone who wants everyone to think that AIPAC is the "Jewish lobby" it's conservatives, because whenever anyone disagrees with AIPAC, the accusations of anti-Semitism are not far behind. As far as they're concerned, the only possible reason you could have a difference of opinion with AIPAC or with the Likud Party it represents is that you hate Jews and want to see Israel destroyed. (And by the way, being Jewish doesn't shield you from these accusations; when Jewish commentators criticize Israel, they get accused of being self-hating Jews.)