One of the key arguments against taking Donald Trump's candidacy seriously is evaporating

Republican presidential front-runner Donald Trump continues to build momentum, both nationally and in key early states like Iowa.

Case in point: A new Bloomberg Politics/Des Moines Register poll of the state, released over the weekend, which provided new reasons to treat his candidacy with seriousness.

Trump is in first place in the poll, grabbing the votes of 23% of likely Iowa Republican caucus-goers. But most striking is how Iowa Republicans now view him in a highly favorable light - after feeling the opposite just three months ago.

According to the poll, 61% of Iowa Republicans now view him either mostly or very favorably, compared with just 35% who see him in an unfavorable light. That's almost directly contrasted with their views in a May poll by the same firm - then, Iowa Republicans viewed him unfavorably by a 63-27 margin.

Trump's favorability ratings have been a key citation in the argument that his candidacy would likely fade quickly - the argument went that he could not expand beyond a rabid, core base that constituted a "ceiling." But polls over the summer have continually disproven that argument. 

In a recent CNN poll of Republicans nationally, for example, 58% viewed Trump favorably - an increase of 8 points from the previous month. He has also boosted those numbers among Republicans in key swing states like Florida, Ohio, and Pennsylvania.

Trump's favorability rating in Iowa is a sign that he's becoming a serious threat to win the first-caucus state. His numbers are on par with those of former Arkansas Gov. Mike Huckabee (R), who won the 2008 Iowa caucuses. He's well ahead of names like former Florida Gov. Jeb Bush (R), New Jersey Gov. Chris Christie (R), and former US Sen. Rick Santorum (R-Pennsylvania), who won the 2012 caucuses. 

That gives him a clear lead with about five months to go before the Iowa caucuses are held in early February. Retired neurosurgeon and fellow political outsider Ben Carson comes in second, continuing his surge and grabbing 18% of the Republican vote. Wisconsin Gov. Scott Walker (R), Sen. Ted Cruz (R-Texas), and Bush round out the top five.

Trump has also cut in half the amount of Republican voters who say they would "never" consider voting for him.

"Like Democrats in 2007 who looked for their savior in Barack Obama, Republicans in 2015 seem to be looking for their savior in Trump, The Des Moines Register's Jennifer Jacobs wrote along with the poll.

If there's someone who should make Trump look over his shoulder, it's Carson, who has surged from 10% in the May poll to 18% in August. Carson is also viewed favorably by a whopping 79% of Iowa Republicans.