Hillary in Iowa

This is the second story I've seen already today (update: rather, make that 3; the previous link, plus the Washington Post and NY Times) commenting on how Bill is coming to Iowa to campaign for Hillary, and of course, by the way, mentioning that Senator Clinton has been strugglig there in the polls, sometimes even coming in third. (here's a summary of the latest Iowa polls of Democrats.) Why is Mrs. Clinton struggling so in Iowa? Is it simply that John Edwards has spent so much time there, really? I wonder.