Seriously, of all the possible reasons to oppose Hagel's nomination, the issue that he MAY be a republican may be the most asinine. But even that is suspect as this interview from this past March showed.
Al-Monitor: Do you still consider yourself a Republican?Right there, he dodges the question and then this coupled with the evidence that he's supported only Democratic Senate candidates like Bob Kerrey, makes me suspect he's no longer a Republican.
Hagel: I don’t know what the Republican Party is. I know why I registered a long time ago; my first vote was in the Mekong Delta absentee in 1968. But we’ll find a new center of gravity... After the American people speak in November, that gives us some new possibilities. If the president is re-elected, like most two-term presidents, your fifth and sixth years can be your most productive.
I know many seem to have intense anger towards him for the remark that he made in the 1990s (which was a popular sentiment among even Democratic politicians) but ignore that. He seems to be a guy who doesn't have a party and simply wants to help his country.
The fact that he is close with President Obama and is of the "realist" foreign policy camp and will implement President Obama's policies (as any cabinet official would) should bolster his candidacy amongst the left not restrain him.