'This Week' Transcript: Secretary of State John Kerry and Ben Carson

— -- THIS IS A RUSH TRANSCRIPT FOR "THIS WEEK" ON DECEMBER 13, 2015 and it will be updated.

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ANNOUNCER: Starting right now on ABC's THIS WEEK, GOP scramble -- the brand new poll out this morning showing national frontrunner Donald Trump falling to second in Iowa.

Is his controversial proposal to ban Muslims helping or hurting the brash billionaire?

And is the GOP establishment starting to panic?

Will Trump go rogue if he's not treated well?

Plus, Ben Carson threatening to leave the party. He speaks out live this morning only on THIS WEEK.

And with the Iowa caucuses 50 days away and two days to the final GOP debate of 2015, this morning on THIS WEEK, so many questions about the race -- answers, insights, analysis. From ABC News, THIS WEEK WITH GEORGE STEPHANOPOULOS begins now.

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GEORGE STEPHANOPOULOS, ABC HOST: And we come on the air this week to a major shake-up in the race for the White House. A brand new poll just released by "The Des Moines Register" shows Ted Cruz blowing by Donald Trump in the key first state of Iowa.

There you see the numbers. Cruz now at 31 percent, 10 points ahead of Trump. The third outsider in this race, Dr. Ben Carson, falling hard from his first place showing in October.

He joins us live in a moment.

But we begin with more on this stunning poll and a defining week in this campaign from Jon Karl at the White House -- good morning, Jon.

JONATHAN KARL, ABC NEWS CORRESPONDENT: Good morning, George.

It is just 50 days until the Iowa Caucuses and while Donald Trump still has a commanding lead everywhere else, he is losing badly to Ted Cruz in the state that gets to go -- vote first. And Republican leaders are gunning for him everywhere else.

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KARL (voice-over): Is this the week Republican Party came undone?

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: It was pretty brutal four days ago for Trump.

KARL: On Monday, Donald Trump announced his most outrageous and controversial plan yet.

TRUMP: Donald J. Trump is calling for a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what the hell is going on?

KARL: Party leaders denounced the proposal as not just wrong, but un-American.

UNIDENTIFIED MALE: What was proposed yesterday is not what this party stands for, and more importantly, it's not what this country stands for.

KARL: Every candidate in the race not named Trump opposed the plan. Jeb Bush even called Trump "unhinged."

TRUMP: I'm not one of these other guys that goes down. I don't go down, I go up.

CHRIS WALLACE, HOST: Do you have a warning for GOP leaders?

TRUMP: I say, folks, you know, I'm sorry I did this to you, but you've got to get used to it. It's one of those little problems in life.

KARL: But now the latest poll out of Iowa shows that Senator Ted Cruz is taking the top spot in the Hawkeye State, 10 points ahead of Trump.

A Trump ally in public, Cruz was caught on tape obtained by "The New York Times" questioning whether Trump has the judgment to be president.

SEN. TED CRUZ (R-TX), PRESIDENTIAL CANDIDATE: Who am I comfortable having their finger on the button?

Now that's a question of strength, but it's also a question of judgment.

KARL: That prompted Trump to take this subtle swipe at Cruz.

TRUMP: I do like Ted Cruz, but not a lot of Evangelicals come out of Cuba, in all fairness. It's true.