Hillary Clinton leads the entire Republican field for President in Iowa, but many of the match ups are closely contested and none of the potential candidates on either side are very popular.
Clinton's leads over the GOP contenders range from 2 to 7 points. The ones who come closest at 2 points are Mike Huckabee and Marco Rubio at 46/44, and Rand Paul at 45/43. The ones who face the biggest deficits at 7 points are Ben Carson, Rick Perry, and Scott Walker at 48/41 and Ted Cruz at 49/42. In between are Jeb Bush who trails by 3 at 45/42, and Chris Christie who trails by 5 at 45/40.
There is a large gap between how Clinton performs and the rest of the Democratic field does in potential general election match ups. Scott Walker leads all of the other hypothetical candidates by margins ranging from 5 to 10 points. It's 39/34 over Sanders, 38/32 over O'Malley, 39/32 over Webb, and 39/29 over Chafee. Those leads are in large part a function of the low name recognition of the other Democratic hopefuls- each of them gets only between 54 and 64% of their own party's vote against the various Republicans with almost twice as many Democrats as Republicans being undecided in each of those contests.