Donald Trump isn't bothered that Nikki Haley indirectly called him angry during the State of the Union rebuttal. 

"I'm very angry because our country is being run horribly. I will gladly accept the mantle of anger," he said.

Image source, Reuters

Ted Cruz defended his background in light of attacks that he is not eligible to run for president because he was born in Canada.

Mr Cruz pointed out that Mr Trump's mother was born in Scotland but he would not bring up his mother's birthplace to attack him.

Mr Trump then said Mr Cruz should "let the courts decide" whether he is eligible to run for president.

"I'm not going to take legal advice from Donald Trump," said Mr Cruz.

They then joked about being each other's vice presidential candidate picks.

Asked how to attack the so-called Islamic State's network, retired neurosurgeon Ben Carson dropped a few grim ideas about attacks of today and the future.

Ted Cruz says the economy is doing well for "Washington lobbyists" and people living in the capital of the US, but not for the average American. 

Candidates have taken the stage for the main debate.

Image source, Reuters

From The New York Times, external:

Donald J. Trump’s popularity has climbed to its highest level to date less than a month before the first Republican votes will be cast, according to a national NBC/Wall Street Journal poll , externalreleased Thursday that shows Republicans becoming increasingly comfortable with the idea of the billionaire businessman being their presidential nominee.

Some key findings:

• Trump is the first choice of 33% of national Republican primary voters

• 65% of Republican voters said they could see themselves supporting him (It was 23% in March)
• But in one-on-one race, Ted Cruz tops Trump, 51 percent to 43 percent

Image source, Reuters

1. Will the gloves come off? Now both vying for front-runner status, Donald Trump and Ted Cruz are starting to attack each other on the campaign trail. Will that carry over to the debate stage?

2. Who leads the "establishment"? Marco Rubio, Jeb Bush, John Kasich and Chris Christie are all after same pool of voters - moderate, pro-business Republicans. 

3. Can Carson come back? Once a top-tier candidate, Ben Carson has seen his fortunes sour in the past few weeks. He recently had a campaign staff shakeup, replacing his campaign manager and top spokesman. Mr Carson promises to be more "lively" now that "the weight has been lifted from his shoulders".

Image source, AP

BBC North America reporter Anthony Zurcher explores front-runner Donald Trump's popularity in Southern states, drawing huge crowds at rallies.

"It all makes for a rather unusual attraction, given that Southerners can look askance at brash Northerners like Trump, whom they often derisively refer to as Yankees." Read more

Since the last debate both South Carolina Senator Lindsey Graham and former New York Governor George Pataki have left the Republican race.

Rick Santorum said the government should give 6 million illegal immigrants the "gift" of mass deportation.

Mr Santorum said he would give illegal immigrants from Latin America "the gift to be able to help the country they were born in" if elected president.

Rick Santorum jokes about taking up some of Kentucky Senator Rand Paul's airtime tonight.

Where is Mr Paul tonight? He is holding an online town hall with Twitter instead of participating in the under card debate. 

Social media is abuzz after this performance at a Donald Trump campaign event on Wednesday night. Lyrics include: "Enemies of freedom face the music, c'mon boys, take them down" and "Deal from strength or get crushed every time." 

Mike Huckabee said that when he went to Afghanistan it looked like the "land of the Flinstones". 

This isn't the first time he's compared the country to a cartoon - he said the same, external in 2011.