Whats wrong with Greenfield?
That is a fair chunk of it; Harkin won most of victories by playing to populism, and even ran for the Populist Party's presidential nomination in '92. Greenfield doesn't strike me as a bad candidate, but I'm not convinced she's a particularly good one, and part of that is completely out of her control (the almost inevitable Republican campaign against the Des Moines Liberal Elite). That said, she did live on a farm in her childhood and is making the occasional nod to that, so is more aware than some previous candidates of the sort of pandering which is probably necessary in a state this rural.
Beyond all that, her biggest weakness seems to be campaigning as generic D. She is not embracing especially populist rhetoric, but could get going when and if she has the nomination. Running as a moderate in IA is all well and good, but it's not particularly effective without significant compromise on a few relevant wedge issues, and I haven't noticed any policy concessions of hers on e.g. guns.
