Sen. Ted Cruz is rapidly gaining support in Iowa, a new poll released Monday finds, with his rise coming mostly at the expense of former neurosurgeon Ben Carson.

Cruz takes 24 percent in the Monmouth University survey , followed by businessman Donald Trump at 19 percent, Sen. Marco Rubio (R-Fla.) at 17 percent, and Carson at 13 percent, with the remainder of the field polling in the single digits.

The results mark a 14 percentage-point rise for Cruz since Monmouth's October poll and a 19-point fall for Carson. Rubio gained 7 points, and Trump's support remained largely unchanged from the previous survey.

Cruz now holds twice Carson's support among Iowa's base of evangelical Christians, who Monmouth estimates will make up about half of the state's GOP caucus electorate. He also has the lead among self-described tea party backers.

Both Cruz and Carson are viewed favorably by two thirds of Republican caucusgoers, representing a modest improvement for Cruz and a significant drop for Carson.

"This marks the first time Ted Cruz has held a lead in any of the crucial early states. As Ben Carsons stock has fallen, Cruz has been able to corral most of those voters, Patrick Murray, the director of the Monmouth poll, said in a statement.

Cruz's headway follows a concerted effort in the state by his campaign. The candidate is trying to make good on a June pledge to visit all 99 of Iowa's counties, according to the Des Moines Register . He's also airing a set of new TV ads that tout his hawkish foreign policy views and an endorsement by Rep. Steve King (R-Iowa), who described Cruz as "the answer to my prayers."

Nineteen percent of the voters surveyed in the Monmouth poll said the endorsement made them at least somewhat more likely to back Cruz, while about three quarters said it wouldn't have an impact.

"Congressman Kings endorsement may not be the primary reason for this swing, but it certainly put a stamp on the Cruz surge in Iowa," Murray said.

Monmouth is the first survey to show Cruz in the lead, but others, including a November Quinnipiac poll that found Cruz taking 23 percent to Trump's 25 percent, have picked up similar trends. HuffPost Pollster's average, which includes all publicly available surveys, shows Cruz and Rubio picking up steam in the state as Carson slumps:

It's far from clear, though, whether Cruz's ascension will be more lasting. Polls more than a month out aren't necessarily predictive of a winner, especially in Iowa's often-volatile caucuses.

A second Iowa poll released Monday also finds Cruz rising, but with Trump still firmly at the head of the pack and continuing to grow. The survey, from CNN/ORC , finds Trump taking 33 percent to Cruz's 20 percent, followed by Carson at 16 percent and Rubio at 11 percent. Trump's share of the vote represents his best showing yet in any survey of the state.

Just 20 percent of voters in the Monmouth poll said they were completely decided on which candidate they'd support, a number that's barely budged since October.

"History shows that even in an normal year, Iowans rarely settle on a final decision until the last possible moment," Iowa pollster J. Ann Selzer wrote Monday for Bloomberg Politics , noting that in 2012, Rick Santorum surged from fourth place to a near-tie for first over the course of her final poll. "Conventional wisdom has it that most caucusgoers have narrowed their choices to three, maybe five candidates at this stage of the race. That appears to be on the low side."

Monmouth surveyed 425 Iowa voters who are likely to attend next year's GOP presidential caucuses, using live interviewers to reach both landlines and cell phones between Dec. 3 and Dec. 6.

The survey differs from past Monmouth polls in the method it used to select which voters get interviewed. The university's past surveys looked only at voters who'd participated in at least one of Iowa's last two state primaries, which attracted some criticism because it screens out first-time caucusgoers, voters who aren't affiliated with a party, and those who vote in presidential but not state-level elections.

In the newest survey, such general election voters comprised 30 percent of the sample, which would suggest "a high-end turnout level of approximately 140,000 caucusgoers."

The pollsters found, though, that the addition had little effect on their results.

"For example, under the previous tighter sampling frame, Cruzs support would be 25% compared to 24% in the full sample, Trumps support would be 16% rather than 19%, and Carsons 13% support would be unchanged," they wrote in the survey's release. "Rubios support using the prior frame would be slightly higher (21%) than in the current frame (17%)."

CNN Polling Director Jennifer Agiesta noted that some of the difference between the Monmouth and CNN/ORC polls likely has to do with voter sampling.

"The CNN/ORC poll drew its sample from Iowa adults, asking those reached about their intention to participate in their caucus, interest in news about the caucuses, and past participation patterns to determine who would be a likely voter," she wrote. "But still, Trump's lead holds even among only those voters who express the most interest in attending the caucus or the most regular past participation in presidential caucuses."

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