About 4 1/2 months before Iowans caucus together one night in February, the candidates who want their votes hosted dozens of events around the state as a new poll showed a race in flux.

Here are seven things we learned from a busy weekend in Iowa:

Warren is the front-runner

Iowa is make-or-break for a growing number of candidates

For all the talk of a nationalized election and a long, drawn-out fight for the Democratic nomination, Iowa is becoming a make-or-break state for a significant number of candidates.

Iowa can launch a president. It can also end a bunch of dreams. This year, more than any previous year, Iowans are going to severely narrow a record-size Democratic field.

Warren is still growing, others aren't

At this weekend's Polk County Democratic Party Steak Fry, a massive picnic that drew every major candidate and thousands of Democratic activists, Warren was one of the few contenders who didn't lead a massive parade onto the pitch. Instead, her team spent time organizing some of the 12,000 attendees who showed up — and, of course, she took selfies. More than a thousand, according to her campaign.

But Warren was one of the few candidates who saw her favorable ratings rise, up 12 points since March.

Biden's favorable ratings have dropped 15 points since March, when he first announced his campaign. The number of Iowa Democrats who see him unfavorably has doubled to 29 percent.

Sanders has fallen too, from a 70 percent favorable rating in June to 58 percent today. His unfavorable ratings, 36 percent, are the highest numbers of any of the top-tier candidates.

Biden's biggest rival isn't Warren, it's Buttigieg

Several candidates have staked their campaigns on the hope that Biden collapses, putting a tranche of moderate Democrats suddenly up for grabs. Bullock, Klobuchar, Bennet and Delaney are circling like buzzards.

But they haven't caught on. Buttigieg has, and he's now the most pressing threat to Biden's campaign.

"Mayor Pete has become the mainstream alternative to Biden," said Jeff Link, a longtime Iowa Democratic strategist who is unaligned this year.

The young mayor is one of only two candidates, along with Warren, whose favorable ratings are higher than Biden's. More than half of Iowa Democrats, 57 percent, say they think someone who represents a new generation of leadership would make a more electable candidate than someone with a long history of serving in government.

And Buttigieg has grown as a candidate more than just about anyone else running this year. At this weekend's Steak Fry, Buttigieg drew some of the loudest and most sustained applause of any candidate, rivaled only by Warren.

Nationally, Biden has the largest share of the Democratic vote. If he falters, and his supporters look elsewhere, he need not fear Warren — he has to worry about Buttigieg, especially if Democratic voters are hungry for the generational change that the first post–baby boomer president would represent.

Warning signs for anyone planning to attack Warren

One sign that Buttigieg increasingly likes his own odds is that he's slowly straying out of his lane and engaging his more liberal rivals.

At this month's debate, he asked why candidates like Warren and Sanders, who back "Medicare for All" and an end to private insurance, don't trust voters to make their own decisions. He repeated that veiled jab at the Steak Fry in Des Moines.

Castro launched a blistering broadside against Biden, suggesting Biden had forgotten elements of his own policy proposals in a thinly veiled shot at the 76-year-old's age. Castro's attack was factually wrong , and politically costly.

Between Selzer's June poll and her September poll, Castro's unfavorable rating shot up from 13 percent to 36 percent. More than half of those who watched the last debate said they had an unfavorable impression of the former San Antonio mayor.

Harris was the first to attack Biden, during the first Democratic debate, about four weeks after Selzer's June poll was conducted. Since that debate, Harris's unfavorable ratings have risen 11 percentage points.

Buttigieg, Biden or anyone lining up an attack on Warren or any other front-runner needs to be wary. No candidate has gotten away with an attack on another candidate without seeing their own numbers suffer as a result.

Democrats are fired up

In the week leading up to the Steak Fry, the most pressing concern was the weather. The forecast called for rain, and threatening clouds sat ominously over Des Moines on Saturday.

But Democrats showed up, in droves. Sean Bagniewski, the chairman of the Polk County Democrats, said the party had sold more than 12,000 tickets at $35 a piece, and they ordered more than 10,000 steaks from Hy-Vee (they also, for the first time, had alternative options on the menu for Booker and Gabbard, the two vegans in the race).

Democratic voters are motivated in the age of Trump. The party won back control of the House of Representatives on the strength of higher-than-expected enthusiasm in 2018. More than 3.3 million small-dollar donors have given a record amount of money to Democratic candidates in the first half of the year, and polls show historic levels of excitement.

The latest CNN survey , conducted by SSRS earlier this month, showed 71 percent of voters are either extremely or very enthusiastic about next year's elections. That's higher than the excitement levels the same poll found just before the 2016, and even just before the 2012 and 2008 elections.

An important caveat to those numbers: About equal numbers of Republicans (51 percent) as Democrats (47 percent) say they are extremely enthusiastic, suggesting the potential for a huge turnout in 2020.

But more immediately, Iowa Democrats are taking heed of turnout at events like the Steak Fry. The party is already planning for the largest-ever caucus turnout . They might want to plan for even more people to show up.

The unanswered questions

Where's Booker's surge? The New Jersey senator has 50 staffers on the ground in Iowa, he gets consistently strong reviews for his rousing stump speech and his favorable rating is higher than all but four others. But he's yet to have a moment on par with Buttigieg's CNN town hall, or Harris's confrontation with Biden, or even Klobuchar's announcement in the snow. And he's running out of time: Booker's campaign took the unprecedented step this weekend of suggesting he could drop out of the race if his numbers don't improve quickly.