Supporters cheer as Republican presidential candidate Donald Trump speaks during a campaign rally, Friday, in Des Moines, Iowa. (Charlie Neibergall/AP)

This post has been updated.

DES MOINES, Iowa -- Donald Trump has dominated nearly every major poll for months, but he has struggled to maintain his lead in Iowa, the first caucus state.

Trump briefly saw retired doctor Ben Carson jump to No. 1 last month, only to plummet. Now, with fewer than two months until the Iowa caucuses, Sen. Ted Cruz (R-Tex.) is posing a threat.

Trump was in Iowa for a rally on Friday night and made clear that he's determined to win -- but here are six things I noticed that hint at the nervousness of the campaign here.

1. Trump slammed a poll that has not yet been released

He warned his audience that sometime soon, the Des Moines Register might release a poll that he might not like.

"Every time the Des Moines Register does a poll, I always do badly," Trump said, then acting out how he imagines the newspaper tallies its poll results and slips some of his votes into a pocket. "'Now, I don't know that they do that. Do you do that Des Moines Register?"

Trump seemed to look into the gaggle of press for a Register reporter to answer his question. There was no one there, as Trump's campaign does not issue credentials to the paper for most events, forcing the reporter covering his candidacy to sneak in with members of the general public. This ban started in late July when the Register editorial board called on Trump to drop out of the race.

An October poll by the Register and Bloomberg Politics showed Carson beating Trump by 9 points, and a late August poll by the newspaper showed Trump in the lead.

Register bashing is apparently popular in Iowa, and one of Trump's biggest applause lines of the night followed this comment: "The Des Moines Register is the worst."

2. Trump started attacking Cruz

For months, these two have had a gentlemanly alliance, defending one another and refraining from attacks. That ended on Friday when Trump questioned Cruz's stances on ethanol and his opposition to the Renewable Fuel Standard, suggesting that the senator's stance was influenced by his ties to major oil companies based in Texas. But through the attacks, Trump kept saying: "Ted Cruz is a nice guy."

[Is this the end of the Trump-Cruz bromance?]

3. Trump now calls himself an evangelical

Evangelical Christians often play a major role in the Iowa caucuses and are aggressively wooed by presidential candidates. Trump has tried to appeal to deeply religious voters by handing out copies of his childhood confirmation photo, declaring the Bible the best book ever written and sharing personal stories about why he doesn't drink or smoke. Friday night he went one step further and declared: "I am an evangelical. I'm a Christian. I'm a Presbyterian."

Cruz has also resonated with evangelicals, recently earning the endorsement of an influential leader of a conservative Christian group. Trump poked at this, making a vague reference to Cuba, the home country of Cruz's father.

"I do like Ted Cruz -- but not a lot of evangelicals come out of Cuba, in all fairness," Trump said. "It's true. Not a lot come out. But I like him nevertheless."

4. There were rows of empty chairs on Friday night

Many of Trump's rallies attract thousands and thousands of people, but Friday night fewer than 1,500 showed up to a cavernous exhibition hall on the state fair grounds. A couple caveats here for Trump fans who will see this entry and immediately go berserk: Yes, that's a much larger crowd than most candidates get on a Friday night two weeks before Christmas. Yes, Trump frequently holds rallies in Iowa, so residents have many more opportunities to see him than residents of most other states. And, yes, the next day, Trump packed a large convention center in South Carolina.

Still, this was Trump's first public rally since Monday, when he called for a temporary ban on most Muslims entering the country -- rows of empty chairs and security checkpoints with no lines were likely not exactly the image the campaign wanted to send.

5. Trump walked back having suggested that Iowans are "stupid"

When Carson was briefly leading in the polls, Trump exploded at Iowans on two separate occasions.

"What the hell are you people doing to me?" Trump asked at a rally in Sioux City in late October. Then during a rally in Fort Dodge in mid-November, Trump questioned why Iowans liked Carson, given parts of his autobiography: "How stupid are the people of Iowa? How stupid are the people of the country to believe this crap?"

[Trump's 95-minute-long rant about everything wrong with this race]

Trump explained to his crowd on Friday night that he was simply "trying to make a point" and did not actually call them stupid. He said the media twisted his words by only reporting his comments on Iowa voters and leaving out what he said about the "people of the country."

"I love you people," Trump said. "Remember that."

6. Trump made clear that winning Iowa is very, very, very important

Trump warned his fans that it's not enough for him to win some primaries -- he needs to lock down as many delegates as possible, otherwise the GOP establishment might try to nominate someone else at the convention. Winning Iowa is an important first step, he said.

"If we win like I think we're going to win -- because we have such a big lead -- honestly, it's not going to matter, they can't do anything. I don't care about the establishment. They can't do anything," Trump said. "If I'm two votes short, I have a problem because I'll have to go through that convention, dealing with all of these bloodsucker politicians. And they'll make their deals, and they'll have all of their money guys around, and they'll be in the backroom making deals. But if I get the number of delegates, there's not a thing that they can do."

[GOP prepares for a contested convention]

Bonus: Trump encouraged his fans to marvel at his Iowa co-chair's weight

I know, I know, this isn't actually a sign of concern for the campaign in Iowa -- but it was too memorable of an exchange to not include. During the rally, Trump praised several of Iowa staffers and Sam Clovis, his Iowa co-chair.

"Where's my big guy? Where the hell is he?" Trump said, scanning the crowd. "Where is he? Sam. Big Sam. Come here, Sam, come here. Look at the size of this. Come here, Sam. Look at him: Big Sam."

Clovis, who often cracks jokes about his weight while warming up crowds in Iowa, came onto the stage.

"Are we going to win Iowa, Sam?" Trump asked.

"We are going to win Iowa," Clovis told the crowd. "We're going to win Iowa! We're going to put them away, we're going to stand on their chest, we're going to step on their throats. We're going to be out of here. We're going to run up the score. We're going to have the biggest victory in the history of the caucuses in the state of Iowa!"

"Beautiful," Trump said. "Beautiful. Thank you, Sam."