Normally, this type of trend line would suggest that Trump’s support on the ballot test would be falling too. Instead, Trump’s 28 percent in the latest Des Moines Register survey was the highest he’s ever earned in that poll. Running a regression on the favorable ratings and ballot test results in the DMR poll reveals a weak relationship — only 21 percent of the variation in a candidate’s polling percentage is explained by his or her favorable rating.

But you can see what’s going on if you break down the favorable ratings by “very favorable,” “mostly favorable,” “mostly unfavorable” and “very unfavorable.” Trump scores a 27 percent “very favorable” rating — just behind Cruz’s 29 percent and Carson’s 28 percent and ahead of Rubio’s 25 percent. The relationship between candidates’ “very favorable” ratings and the ballot test is far stronger than when you combine the “very favorable” and “mostly favorable” ratings — the “very favorable” ratings explain 68 percent of the variation in the ballot test numbers.

Trump himself has seen little change in his “very favorable” rating. Since August, it’s consistently been between 25 percent and 27 percent. The two candidates who have overtaken Trump in Iowa polls since August, Carson and Cruz, have seen much more volatility in their “very favorable” ratings: Carson’s has ranged from 28 percent to 53 percent, and Cruz’s from 28 percent to 43 percent. Both are near their low points for the period in the latest Des Moines Register survey.