A new Quinnipiac University poll finds businessman Donald Trump has the support of 25 percent of likely Iowa Republican Caucus goers and Texas Senator Ted Cruz has 23 percent. Peter Brown, the assistant director of the poll, says support for Cruz has more than doubled in the past four weeks.

“Senator Cruz is the hot candidate in Iowa. There’s no doubt about that,” Brown says. “That’s a very big jump in just one month.”

Retired surgeon Ben Carson is in third place, but his support in Iowa dropped by 10 percent in the past month — while Cruz picked up 13 points. Brown says those trends are “mathematically linked.”

“Many of these are the same people,” Brown says. “Not all, necessarily, but many.”

The poll was taken from November 16th through the 20th — after the terrorist attacks in Paris — and only six percent of those surveyed gave Carson high marks on foreign policy experience, while Cruz topped the chart as the candidate judged “best able to handle” foreign affairs.

“Dr. Carson’s background which is relatively light on foreign policy experience and Senator Cruz’s relative depth, you know, it’s not terribly surprising there’s been this movement,” Brown says. “Also Senator Cruz seemed to get the best grades out of the last televised debate, so that’s helpful.”

Support for Florida Senator Marco Rubio held steady in the month-to-month comparison. Brown says the GOP candidates are now clearly separated into two tiers.

“There’s the first tier, which is made up of four candidates — two insiders, Senators Cruz and Rubio; and two outsiders, Dr. Carson and Mr. Trump — and everybody else is pretty far back,” Brown says. “This is Iowa, so anything’s possible, but for someone to move from that bottom tier into that top tier is going to take an awful lot of work and good luck.”

Thirty percent of likely Iowa Republican Caucus-goers listed “terrorism” and “foreign policy” as their top issues, while 24 percent said “jobs and the economy” are their number one concern.