One of the main arguments against taking Donald Trump's campaign seriously keeps evaporating

Donald Trump's image with Republican primary voters has seen a sharp positive uptick over the past two weeks, adding to new signs of his durability as a candidate.

New Gallup data released Friday shows that Trump's net favorability rating - the difference between the percentage of people who view him favorably vs. unfavorably - jumped 16 points in just the past couple of weeks.

He's now viewed favorably by 63% of Republican voters nationwide, compared with 31% of Republicans who have an unfavorable opinion of him. The 32-point net favorable rating puts him behind five other GOP candidates, a far cry from when he was routinely one of the most disliked candidates in the Republican field.

The most popular Republican candidate by Gallup's methodology is retired neurosurgeon Ben Carson, who has rocketed up polls alongside Trump following the first Republican presidential debate last month. He is viewed favorably by 57% of Republicans, while just 6% have a negative opinion of him.

But while voters' opinions of Carson have improved as he has stepped into the national spotlight, Trump has defied past political trends by reversing a deeply unpopular sentiment about him. For example, one poll of Iowa voters in May found that the Hawkeye State's Republicans viewed him unfavorably by a 63-27 margin. Now, they see him favorably by a 61-35 split.

"This marks a significant shift; Trump's image previous to the last two weeks had been relatively stable despite the extraordinary media attention his candidacy has engendered," Gallup editor in chief Frank Newport wrote.

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 disproved 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, as well as the first-caucus state of Iowa.

Aside from Carson, the GOP candidates with better favorability ratings among Republicans than Trump are Sen. Marco Rubio (R-Florida), Sen. Ted Cruz (R-Texas), former Arkansas Gov. Mike Huckabee (R), and former Hewlett Packard CEO Carly Fiorina.