‘This Week’ Transcript: Donald Trump and Sen. Marco Rubio

— -- THIS IS A RUSH TRANSCRIPT FOR “THIS WEEK” ON SEPTEMBER 20, 2015

(BEGIN VIDETOAPE)

ANNOUNCER (voice-over): Starting right now on ABC, THIS WEEK, Trump under fire. The GOP front-runner taking heat for what he didn't say. The bombastic billionaire is here, revealing why he's not backing down.

Plus: after the big GOP showdown, could new stars emerge to shake up the race? We'll ask Marco Rubio.

And Hillary Clinton's back on offense.

HILLARY CLINTON (D), PRESIDENTIAL CANDIDATE: Unbelievable. It's amazing.

ANNOUNCER (voice-over): Dancing on the rope line with reporters.

Plus the growing debates over the Democratic debates. Should there be more?

And the pope makes history. His landmark trip begins and we're right there with him as he makes his way to the U.S. for the first time.

From ABC News, THIS WEEK with George Stephanopoulos begins now.

(END VIDEOTAPE)

GEORGE STEPHANOPOULOS, ABC HOST: Good morning. You are looking live at Cuba, Pope Francis there this morning ahead of his first trip to the U.S. this week. We're going to have more on that later in the show.

But we being with the race for the White House after a second record-setting debate this week, Carly Fiorina on the rise; big night, too, for Marco Rubio and Donald Trump, now facing new questions. We're going to talk to Trump and Rubio live.

First, Jon Karl on the latest controversy Trump is stirring.

(BEGIN VIDEOTAPE)

JONATHAN KARL, ABC NEWS CHIEF WHITE HOUSE CORRESPONDENT (voice-over): Even after a rough-and-tumble campaign week, Donald Trump is still the king.

UNIDENTIFIED MALE: And the future President of the United States of America, Donald Trump.

KARL (voice-over): Welcomed last night by the homecoming court at an Iowa high school.

TRUMP: I love you all. Have a great dance tonight.

KARL (voice-over): But the latest Trump firestorm is not over something he said but what he didn't say when faced with this inflammatory question in New Hampshire.

UNIDENTIFIED MALE: … problem in this country. It's called Muslims. We know our current president is one. You know he's not even American.

TRUMP: We need this --

(CROSSTALK)

UNIDENTIFIED MALE: -- this man.

KARL (voice-over): Trump didn't challenge his supporter's false claim about the president's background.

GOV. CHRIS CHRISTIE (R-NJ), PRESIDENTIAL CANDIDATE: If someone brought that up at a town hall meeting of mine, we'd say, no, listen, before we can answer, let's clear some things up for the rest of the audience. And I think you have an obligation as a leader to do that.

KARL (voice-over): But Trump has no apologies. He recited his Twitter account for an Iowa crowd on Saturday.

TRUMP: So I started by saying, "Am I morally obligated to defend the president every time somebody says something bad or controversial about him? I don't think so."

Right?

KARL (voice-over): So is Donald Trump playing with fire? Or simply playing to his base?

A recent poll shows more than half of Trump's supporters believe President Obama is Muslim and 28 percent of Republicans think the president wasn’t born in the U.S. Even Trump has admitted his conspiracy theories over the president's birth are part of his appeal, as he told me two years ago.

(BEGIN VIDEO CLIP)

KARL: You don't acknowledge that you went overboard on this whole birther stuff?

(CROSSTALK)

TRUMP: Actually, I think it made me very popular, if you want to know the truth, OK? So I do think I know what I'm doing.

(END VIDEO CLIP)

TRUMP: More energy tonight, I like that.