President Obama is reportedly considering nominating Republican Chuck Hagel to run the Pentagon. With no offense to the former Senator, there is no reason for the President to look outside of his party for a qualified Secretary of Defense.
Democratic presidents have frequently used moderate Republicans in Defense and Intelligence posts. This signals falsely to the public that Democrats are not qualified to lead on these issues. Democrats have plenty of talent in this regard and can better be counted on to govern based on the values of the party chosen to lead the executive branch.