I've long held the view that demographics drive elections - and especially presidential elections - more than the campaigns do. So, heading into the 2016 presidential campaign, Republicans are in a bit of trouble as the demographic trends continue to move against them. Indeed, an argument could be made that the climb for Republicans will get even steeper as a new Harvard study finds that race probably cost President Obama about 4 points in the 2008 and 2012 elections. Even discounting that study (and there are good reasons to that I won't bother covering here), the Republican Party has well-documented problems with various demographic groups from African-Americans to Latinos to women to young people and so on. Indeed, the base of the Republican Party is well ... old, white guys. And there aren't enough of them for Republicans to win.
Republicans should be worried about Hillary because she's strong in demographic groups where Obama isn't/wasn't. Hillary isn't going to beat any Republican among old, white guys. But she's a very good bet to run a little bit stronger than Obama among them. She's a sure-fire bet to run better among women - a group Obama won but not as handily as a generic Democrat might have. Finally, for those voters who worried about Obama's lack of foreign policy credentials in 2008, Hillary's credentials are unrivaled among all candidates in the field, Republican or Democratic.
If the 2016 election were held today, a generic Democrat would handily beat a generic Republican. But actual Republican candidates have good reason to worry about Hillary as they're even further behind her right now.