Donald Trump simply doesn't understand why a series of polls have shown him losing in Iowa to retired neurosurgeon Ben Carson.

Trump has held several rallies that attracted "tremendous crowds" of exuberant Iowans, while Carson is rarely spotted there. Trump will hold a rally in Sioux City in western Iowa on Tuesday night, his second in the state in less than a week. Meanwhile, Trump's campaign has 13 staffers working in Iowa, along with a network of volunteers, making it the largest Republican operation in the state. And the campaign has been carefully building its own database of potential caucus-goers, a large number of whom they believe don't usually vote.

Trump was last in Iowa last Wednesday for a rally in Burlington, an industrial river town in the eastern part of the state. Since then, his lead in the state has vanished. On Thursday, a Quinnipiac University poll reported that 28 percent of likely Iowa caucus participants would vote for Carson, compared to 20 percent for Trump. The next day, a poll from the Des Moines Register and Bloomberg Politics put Carson again at 28 percent and Trump at 19. A CBS-YouGov poll on Sunday showed Carson and Trump tied at 27 percent each -- but two more polls on Monday showed Carson with a double-digit lead over Trump.

"I don't get it," Trump said on MSNBC's "Morning Joe" on Tuesday. "Some of these polls coming out, I don't quite get it. I was No. 1, pretty much, in Iowa from the beginning, and I would say we're doing very well there. So I'm a little bit surprised. The other polls, as you know, in other states are extraordinary, actually. But this one I don't quite get. I would have thought we were doing much better -- I think we are doing much better."

Trump said in the interview that he has no plans to write Iowa off: "I'll fight right to the end for it," he said. "And if I don't win it, I understand that can happen, but I will be fighting for Iowa."

Ben Carson has placed first in a national Republican presidential primary poll, pushing Donald Trump into second place for the first time since June. Deborah Lutterbeck reports. (Reuters)

And a New York Times-CBS poll released Tuesday showed Carson leading nationally at 26 percent, with Trump at 22 percent. Trump often devotes large chunks of his campaign speeches to listing off his latest poll numbers, citing both scientifically and unscientifically collected results. Now that he's slipping, skepticism has descended.

"The thing with these polls, they're all so different," Trump said. "One guy is up here, somebody else is up there, you see swings of 10 and 12 points and, immediately, even the same day. So right now it's not very scientific."

Trump said he hopes Carson's frontrunner status will bring some scrutiny of the doctor's stances and leadership capabilities because he sees "a lot of contradiction and a lot of questions." Trump accused Carson of being "pro-abortion not so long ago" -- even though Trump's own stances on abortion appear to have evolved over the years, having described himself as "very pro-Choice" in 1999. Trump criticized Carson for proposing to change the health-care system in a way that would abolish Medicare and questioned how Carson would negotiate with foreign leaders.

Trump knocked Carson for spending a "tremendous amount" on his campaign, including hiring staff and paying people to raise money for him. After bragging about his campaign's frugal spending, Trump said he is "willing to spend whatever it takes" to win.

These sorts of attacks are expected to continue Tuesday night as Trump takes the stage for a rally at a high school in Sioux City. A group of pro-immigration activists have questioned the school for allowing Trump to appear there and plan to protest outside the event.

What Donald Trump is doing on the campaign trail

U.S. Republican presidential nominee Donald Trump speaks at a campaign event at Trump Doral golf course in Miami, Florida, U.S. July 27, 2016. REUTERS/Carlo Allegri (Carlo Allegri/Reuters)