It’s hard to imagine anyone thinking that Iowa voters’ party affiliation this time around won’t be at least somewhat to the right of where it was in 2008 — in other words, somewhere to the right of a 1-point Democratic edge. How far to the right of that is harder to guess. But if we assume that party affiliation is dead-even nationally — which is slightly to the left of what Gallup indicates (Gallup shows Republicans with a 1-point edge) — and if we assume that Iowa’s party-affiliation split will again be 6 points to the right of the nation as a whole, that would mean a GOP turnout advantage in Iowa of 6 points this time around. If, however, Iowa is only half as far to the right of the nation in 2012 as it was in 2008 (so, 3 points), or if it remains about 6 points to the right but Democrats actually have, say, a 3-point edge nationwide in party affiliation (meaning Gallup is underselling Democrats by 4 points), then the GOP edge in party affiliation in Iowa would be about 3 points.
Either way — whether Republicans will have a 3-point or 6-point edge in turnout in Iowa (or even anywhere between 2 and 7 points) — that’s pretty close to Rasmussen’s projection of a 4-point GOP edge in turnout, and it’s a long way removed from the projections from NBC/WSJ/Marist (Democrats by 3) or Gravis (Democrats by 6).