A new Pew poll shows that there's no longer any difference between Democrats and Republicans on Iraq: huge majorities agree that the war was a failure.
What's interesting is the inflection point in 2008: Democrats became suddenly more optimistic about Iraq and Republicans became more pessimistic. This was before Barack Obama won the election, so it's not directly because of that. But by mid-2008, negotiations over withdrawal had stalled and it was clear that the end of the US troop presence was near. It was also increasingly clear that Obama was likely to win the presidency. Those two things combined might account for the partisan differences.
By 2012, with US troops gone, those partisan differences started to disappear. By 2014, they were gone. Hardly anyone could fool themselves into thinking that the Iraq War had succeeded in any way: there were no WMDs; there wasn't much oil flowing; Iran's influence had increased; and sectarian violence was once more on the rise. A third of the country can still be described as dead-enders on this score, but that's it. Everyone else has finally faced the facts.