Six months ago, Walker led Iowa
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The Iowa caucus landscape has shifted dramatically over the last six months, leaving the GOP field almost unrecognizable from where it sat before the “Summer of Trump.” 
Donald TrumpDonald TrumpMcMaster to South Korea: US will pay for missile defense system Comedian Hasan Minhaj blasts Trump, media at WHCA dinner White House correspondents' chief: 'We are not fake news' MORE rocketed to first place in polls of Iowa voters in August, driving past Wisconsin Gov. Scott Walker, who was soon out of the GOP presidential race.
Since then, Trump has been the most consistent factor in the GOP race and in Iowa in a tumultuous year for the Republican Party.
Walker had been seen as the early favorite for the caucuses after he burst onto the 2016 scene with a much-lauded speech at the Iowa Freedom Summit in February.
He led every poll released between mid April and July, according to data compiled by RealClearPolitics, and his campaign mounted an aggressive Iowa-first strategy when he declared in July.
But August marked the beginning of the end for Walker, and the beginning of the Trump dominance. Trump overtook Walker at the polls just days into August and Walker fled the race just months later as his fundraising dried up. Since then, the real estate magnate has cemented his status as the GOP frontrunner with an almost unencumbered reign at the top of Iowa's polls. 
Two candidates have made serious runs at Trump’s lead since, bolstered by success with the large evangelical segment of Iowa’s GOP caucus voters. But only one has been able to leverage that run into a strong position on caucus night.
After sitting back for months and starting August in just sixth place, Ted CruzTed CruzTrump in campaign mode at NRA convention Trump’s hands are tied on 9th Circuit Schumer: Trump's handling of North Korea 'all wrong' MORE barnstormed his way to the lead in Iowa by December. His reign of almost one month at the top of the polls has drawn the ire of Trump, who has targeted Cruz with a series of attacks on his Canadian birth.
The Texas senator’s coalition of evangelical, libertarian and Tea Party supporters helped him peak at the right time, putting him in solid position to snatch a win from Trump.
Ben Carson had a similar rise, but a much steeper fall. Polling in second for a large part of the summer and fall, Carson became the first candidate to knock Trump off the top of the Iowa totem poll.
Carson emerged as a strong foil to Trump, as his mild-mannered approach has just about nothing in common with Trump’s approach.
But just weeks later, Carson plummeted thanks in large part to a series of foreign policy missteps. He dropped more than 10 percentage points in December and now sits in a distant fourth place.
In between Carson and Cruz is Marco RubioMarco RubioOvernight Defense: Commander calls North Korea crisis 'worst' he's seen | Trump signs VA order | Dems push Trump to fill national security posts What’s with Trump’s spelling mistakes? Boeing must be stopped from doing business with Iran MORE. The Florida senator has picked up the pace in Iowa down the stretch, and hopes a third-place finish could make voters see him as the most electable candidate in a crowded race.
The rest of the field has also seen a changing of the guard since August.
Former Florida Gov. Jeb Bush was in third place according to the RCP average of polls on Aug. 1, but has now slipped to sixth place, more than 24 points behind Trump.
In the spring, former Arkansas Gov. Mike Huckabee was a leader in Iowa, but he is now behind Bush and largely an afterthought in the race.
Former HP CEO Carly Fiorina had a brief surge in Iowa after a strong performance in August’s GOP undercard debate. She polled as high as third place in Iowa before falling back in late October into the lower tiers of the field.
She now stands at ninth place in the RCP average with 2.6 percent.