If the caucuses were held today, Cruz would win with 24% support to Trump's 19% backing. Marco Rubio
comes in a close third with 17% and Ben Carson
, who was beating Trump 32%-18% in the last Monmouth University poll, has fallen to 13% support.
Behind the Cruz surge is support among evangelicals -- a decisive voting bloc in Iowa -- who selected Cruz over Trump 30%-18%.
Monmouth poll director Patrick Murray attributed Cruz's rise to Carson's fall in the early nominating state, but also noted that Iowa Rep. Steve King's endorsement likely had an impact. Nearly 20% of respondents said that King's endorsement made them more likely to support Cruz.
"This marks the first time Ted Cruz has held a lead in any of the crucial early states. As Ben Carson's stock has fallen, Cruz has been able to corral most of those voters," Murray said in a statement Monday. "Congressman King's endorsement may not be the primary reason for this swing, but it certainly put a stamp on the Cruz surge in Iowa."
The Monmouth poll was conducted from December 3-6 with 425 likely Republican caucusgoers and carries a +/- 4.8% margin of error.
The poll suggests Trump may be able to overtake Cruz, however, if he is able to draw new Republican supporters to the typically low-turnout caucuses.
The poll was conducted among registered voters, with the bulk of the interviews coming from those who are regular Republican primary voters. Monmouth's pollsters report that those who are not regular Republican primary voters were more likely to support Trump, while among those who had participated in previous primaries, Cruz held an edge.
Iowa allows participants in the caucuses to register at any time before the caucus, including on-site just before it begins.