Over the last several weeks, the story of the Iowa caucuses has become the battle between Donald Trump and Ted Cruz.

For months, the two outsiders appeared to have struck an alliance, as they beat back their opponents and rose to the top of the Republican field—only to turn on each other in early January and launch a succession of brutal attacks. Cruz’s 10-point lead over Trump in December’s Bloomberg Politics/Des Moines Register Iowa Poll has become a casualty of that battle: in the new Jan. 30 survey, Trump vaulted past Cruz and to take a 28 percent to 23 percent lead.

New data from the latest poll, and interviews with the candidates’ campaign managers, shed light on the strategy, and the impact, of the clash that could decide the Iowa caucus winner on Feb. 1—and also strongly influence the course of the Republican nomination process.

Rattled by Cruz’s growing strength in Iowa, Trump began raising the issue of Cruz’s Canadian birthplace in early January, claiming it could disqualify him from the White House if he didn’t meet the Constitution’s requirement that “no person except a natural born Citizen … shall be eligible to the Office of President.” Cruz initially shrugged off the charge. Most legal scholars thought he was eligible. But several did not. And the attack registered. In the Jan. 13 Iowa Poll, 15 percent of likely Republican caucus-goers said they were “bothered” that Cruz “was born outside the United States.” But after two more Republican candidates, Rick Santorum and Mike Huckabee, picked up the theme, Cruz’s trouble spread. In the latest poll released just two days before the caucuses, 22 percent of respondents were “bothered” by Cruz’s former dual citizenship. Sensing blood, Trump sharpened the attack. At a rally on Friday, he called Cruz “an anchor baby in Canada.”

“Donald Trump has instincts that have guided his professional career for the last 40 years,” Corey Lewandowski, Trump’s campaign manager, said of the decision to key on the “birther” issue. “You don’t become one of the wealthiest people on the planet without being able to read people and understand them. And that’s what he’s been able to do [with Cruz].”

Trump didn’t limit his attacks to Cruz’s Canadian origins. On ABC’s This Week, he also asserted that Cruz was a “very nasty guy.” As Trump explained on national television: “Nobody likes him. Nobody in Congress likes him. Nobody likes him anywhere once they get to know him.” Trump also called Cruz a “total hypocrite” for taking out a loan from Goldman Sachs and failing to disclose it properly.

Both attacks landed. In the final Iowa Poll, 33 percent of respondents were “bothered” that Cruz’s “personality puts a lot of people off.” More than half of respondents (54 percent) were bothered that Cruz “borrowed up to $1 million from Wall Street banks,” a particularly effective line of attack against the Texas senator. Says J. Ann Selzer, who conducted the poll for the news organizations, “Our data would say that the thing that bothers people the most [about Cruz] is the Goldman Sachs loan” for up to $500,000 that Cruz initially did not disclose to the Federal Election Commission as required—a recent revelation reported by The New York Times that Trump may not yet have fully exploited.

Trump also appears to have profited from winning Sarah Palin’s endorsement—an implicit rebuke to Cruz, who credits Palin for his 2012 Senate victory. Although Palin’s favorability rating in the new poll is only middling (37/50 favorable/unfavorable), she is much stronger with those who consider themselves “very conservative” (48 percent favorable) and “Tea Party” voters (50 percent favorable), two groups upon whom Cruz relies heavily. On Jan. 19, Palin endorsed Trump in Ames, Iowa, which lies in the heavily evangelical 4th Congressional District. Trump now leads Cruz in the 4th District by 27 percent to 26 percent. “In theory, that could be Sarah Palin,” says Selzer.

This barrage of attacks on Cruz, along with those from Santorum, Huckabee, U.S. Senator Marco Rubio, and GOP luminaries such as Iowa Governor Terry Branstad and former Senator Bob Dole, have tarnished Cruz’s once-sterling image among Republicans and eliminated his lead in the poll. Cruz’s favorability rating fell 11 points in January, from 76/19 favorable/unfavorable in the Jan. 13 poll to 65/28 in the latest one.

Cruz eventually responded. After spending the last six months of 2015 mostly praising Trump and serving as a character witness, Cruz finally changed tack in early January, assailing Trump’s “New York values.” He also began highlighting a barb Trump issued at a November rally in Ft. Dodge, Iowa, when he wondered aloud “How stupid are the people of Iowa?” for believing Ben Carson’s claim that a belt buckle had once prevented him from being stabbed by a childhood friend.

But Trump’s favorability ratings have fallen only modestly as a result, from 54/45 favorable/unfavorable on Jan. 13 to 50/47 on Jan. 30. And during this time, Trump’s overall percentage of support has broken through what had appeared to be a ceiling in the low 20s.

The question haunting the Cruz campaign is whether the Texas senator waited too long to hit back at Trump after he first raised the “birther” issue on Jan. 5. Cruz initially laughed off the attacks through his Twitter feed, suggesting that Trump had “jumped the shark.” His campaign initially withheld comment in order to deprive Trump’s charge of oxygen. According to several of his strategists, Cruz also wanted to avoid getting into the sort of war with Trump that has doomed other candidates. Yet Trump kept pounding away.

After nine days, the Texan finally swung back hard during the Jan. 14 debate, calling Trump’s move an act of desperation with “zero” basis. Since then the two front-runners have been locked in a hammer-and-tongs fight. In another finding in the new Iowa poll, Cruz has several promising lines of attack that he could choose to amplify. One sleeper issue might be Trump’s support for eminent domain, which “bothered” 60 percent of likely caucus-goers. And Trump’s past support of abortion rights, including his unwillingness to ban late-term abortions, is nearly as salient: it bothers 56 percent of caucus-goers.

But one challenge for Cruz will simply be to secure some breathing room from Trump. “He seems to have a lot of cards to play,” says Selzer. “It feels like a well-orchestrated approach to keeping his name in the news—and everybody else’s name out of the news.”

Cruz’s campaign manager Jeff Roe said his team’s internal polls don’t show the candidate’s favorability ratings dropping and insisted that Trump’s attacks haven’t made a dent. “I’ve seen no piece of data—people don’t believe that Senator Cruz is an ineligible candidate for president,” he said on Thursday night in Des Moines after the debate. On Friday morning, Roe conceded that the “birther” attack had cost Cruz some supporters, but said it was immaterial because the candidate’s overall support grew during January.

Asked if Cruz waited too long to push back against Trump’s attacks, Roe demurred.

“We’ll find out on Monday,” he said.

The poll of 602 likely Republican caucus-goers was conducted Jan. 26-29 by Selzer & Company of West Des Moines, Iowa. The margin of error on the full sample is +/- 4.0 percentage points. 

(Corrects details of Cruz's loan from Goldman Sachs starting in the sixth paragraph. He received up to $500,000 in loans from Goldman, and initially disclosed it in a personal filing but not in one to the Federal Election Commission, according to The New York Times.)