Donald Trump has overtaken Ted Cruz in the final days before Iowa's caucuses, with the fate of the race closely tied to the size of Monday evening's turnout, especially among evangelical voters and those attending for the first time, a Bloomberg Politics/Des Moines Register Iowa Poll shows.

The findings before the first ballots are cast in the 2016 presidential nomination race shows Trump with the support of 28 percent of likely caucus-goers, followed by 23 percent for the Texas senator and 15 percent for U.S. Senator Marco Rubio of Florida.

The billionaire real estate mogul leads Cruz among those who say they definitely plan to attend, 30 percent to 26 percent. With the less committed—those who say they'll probably attend—Trump also beats Cruz, 27 percent to 21 percent.

“Trump is leading with both the inner core of the caucus universe and the fringe—that’s what any candidate would want," said longtime Iowa pollster J. Ann Selzer, who oversaw the survey for the news organizations. 

Read the poll questions and methodology here.

The poll's findings are based on 47 percent of those likely to attend considering themselves evangelical or born again Christians. When re-weighted as a scenario test for the higher evangelical turnout seen in 2012 entrance polls, the race is closer, with 26 percent for Trump and 25 percent for Cruz.

A Trump victory could significantly boost his chances of winning his party's nomination, while a second-place finish for Cruz would be a major setback for a candidate who has invested heavily in Iowa and enjoyed strong support from evangelical Christians who form a large part of the state’s electorate. Trump is dominating in polling in New Hampshire and South Carolina, the two states that follow Iowa in the nomination calendar.

Just two days before the first-in-the-nation caucuses, the race remains fluid, even after hundreds of campaign stops in Iowa, tens of millions of dollars of advertising and seven nationally televised debates.

More than half—55 percent—say their mind is made up, while 45 percent say they either don't have a first-choice candidate or could still be persuaded to pick someone else. In the final Iowa Poll before the 2012 Republican caucuses, 51 percent say they had their minds made up.

Trump's advantage over Cruz is a reversal of the race in the previous Bloomberg Politics/Des Moines Register Iowa Poll in early January, when he trailed 25 percent to 22 percent.

Under near constant attack from Trump since December, Cruz’s favorability rating has also dropped—by 11 points to 65 percent. Trump is viewed favorably by 50 percent, a four-point drop since the prior poll and the lowest of the top four candidates. Retired neurosurgeon Ben Carson and Rubio are in the low 70s.

The crowded Republican field appears to be working against Cruz. If the race for the nomination eventually became a two-person race between Trump and Cruz, 53 percent of likely Republican caucus-goers would pick Cruz, while 35 percent would go with Trump.

"There's an appreciation for Cruz even among people who are voting in a different way," said Selzer, who is widely considered the state's top pollster. "For Trump, he might be able to win, in part because the field is as big as it is."

In fourth place is Carson, who is backed by 10 percent of likely Republican caucus-goers, followed by U.S. Senator Rand Paul of Kentucky at 5 percent. No other candidate recorded above 3 percent.

At the time of his departure from the race in September, Wisconsin Governor Scott Walker called on other candidates to also drop out, so that an establishment candidate could emerge with enough support to challenge Trump. Yet the combined support of candidates in that lane—Rubio, former Florida Governor Jeb Bush, Ohio Governor John Kasich and New Jersey Governor Chris Christie—is still less than Trump or Cruz. 

Support for Rubio, who has emerged as the leading establishment candidate, remained flat as the caucuses near. In fact, over the four days of the survey, his support dropped the last two days.

Supporters of Trump are the most decided among the top three candidates, with 71 percent saying their mind is made up, compared to 61 percent for Cruz and 47 percent for Rubio. Trump also leads with most demographic groups measured by the poll, including those without college degrees, moderates and Catholics.

The poll's findings suggest Trump is inspiring new interest in the Republican caucuses: 40 percent of those in the survey say they'll be attending for the first time, the highest number recorded by the survey this election cycle. The last Iowa Poll before the 2012 caucuses showed 27 percent first-time caucus-goers.

Nearly one-third of Cruz’s supporters say they’d be attending for the first time, compared to half of Trump's supporters who say they'll be going for the first time, suggesting he has a greater challenge in turning out his supporters because veteran caucus-goers tend to be more reliable.

Cruz's drop from an Iowa Poll in early December—when he led the field at 31 percent—reflects a falloff in support across multiple demographic groups, including people who define themselves primarily as evangelical conservatives, where his backing dropped 12 percentage points. His support among the youngest and oldest also dropped and he lost 14 points in the Third Congressional District that includes parts of central and southwest Iowa, including the state capitol of Des Moines.

The poll suggests there could be growing Trump fatigue as well in a state he's visited often in recent months. Almost half of those likely to attend the Republican caucuses say they've become less comfortable with the idea of him winning the nomination, while 49 percent say that of his prospects of representing the U.S. to the rest of the world and 45 percent on his possibility of winning the presidency.

Likely caucus-goers are becoming more comfortable, meanwhile, with the prospects of Cruz doing those things, with roughly half saying they've moved that direction.

On candidate traits tested, Trump won on almost every question. He beats Cruz on being most feared by U.S. enemies (50 percent to 21 percent), potential to bring about needed change (37 percent to 21 percent), being a strong leader (32 percent to 23 percent), prospects for winning a general election (35 percent to 24 percent) and keeping "your family safest" (28 percent to 24 percent).

Cruz beats Trump on having the "greatest depth of knowledge and experience" (26 percent to 17 percent), as well as being respected by leaders of friendly countries (20 percent to 16 percent).

Two dramatic moves in the final weeks of the Iowa race appeared to make little difference. A plurality—46 percent—say they didn't care that Trump skipped the debate in Des Moines this week, while Iowa Governor Terry Branstad’s plea to defeat Cruz failed to sway 77 percent of likely Republican caucus-goers.

In a test of some other lines of attack that have been used against Cruz and Trump, the poll found varying strength.

Three-quarters of likely Republican caucus-goers say they're not bothered by the fact that Cruz had dual citizenship with Canada until recently, while almost two-thirds don't care that Cruz's personality is off-putting to some, including Senate colleagues from both parties.

A slim majority of 54 percent say they're bothered that Cruz failed to fully disclose up to $1 million in loans from Wall Street banks during his 2012 Senate run. Less than half—43 percent—were bothered by Cruz's position on the Renewable Fuel Standard.

Trump’s support for the use of eminent domain to take, in exchange for compensation, private property for public or private projects, was a concern for 60 percent of those polled. Nearly as many—56 percent—are bothered that in the past he supported abortion rights and said he wouldn't ban late-term abortions.

More than six in 10 aren't bothered by questions about Trump’s familiarity with the Bible, while almost two-thirds aren't bothered that some of his business have filed for bankruptcy.

With former New York Mayor Michael Bloomberg considering an independent presidential bid, the poll tested his favorability ratings among likely Republican and Democratic caucus-goers.

The findings highlight some of the hurdles facing Bloomberg should he decide to enter the race. The former mayor isn't well known in the state among the most motivated voters in both parties, with 41 percent of likely Republican caucus-goers and 57 percent of likely Democratic caucus-goers not knowing him well enough to share an opinion.

For those who do have an opinion, 50 percent of Republican caucus-goers have an unfavorable view, versus just 9 percent who hold a favorable opinion. Among those likely to attend the Democratic caucuses, that split was 26 percent unfavorable to 17 percent favorable.

Bloomberg was a three-term mayor of New York, twice as a Republican and finally as an independent, and is the founder and majority owner of Bloomberg LP, parent of Bloomberg News.

The survey, conducted Jan. 26-29 by Selzer & Co. of West Des Moines, Iowa, included 602 likely Republican caucus participants and 602 likely Democratic caucus participants. It has a margin of error of plus or minus 4 percentage points.

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