CHARLESTON, SC — (CNN) The field of candidates fighting for the Republican Party’s presidential nomination is dwindling, leaving seven to compete in the prime-time GOP debate hosted by Fox Business Thursday night.

Less than three weeks away from the February 1 Iowa caucuses, front-runner Donald Trump will once again take the center podium. He’ll be flanked by Ted Cruz, his biggest threat in Iowa, and Marco Rubio, who is trying to unite the establishment behind him and fire up Republicans looking for a non-Trump or Cruz option.

Three candidates who are running on their records of governing big states — Chris Christie, Jeb Bush, and John Kasich — will also be on stage, in one of the last prime-time opportunities to shape their narratives and boost their numbers before voting in Iowa begins.

And three of the lowest-ranking GOP candidates will participate in the undercard debate earlier in the evening: Carly Fiorina, Mike Huckabee and Rick Santorum. Like Fiorina, Rand Paul failed to make the cut for the main event, but he is declining to participate.

Here’s what to watch for Thursday night:

Can Cruz really keep ignoring Trump?

Ted Cruz has exercised a rare kind of self-restraint during the 2016 campaign: He’s refused to attack Donald Trump.

Trump, leading nationally but trailing Cruz in some Iowa polls, has consistently taunted his rival. He’s challenged Cruz’s stance on ethanol subsidies — a critical issue in Iowa where farming and agriculture are influential industries. He’s tweaked Cruz on religion, suggesting that the Southern Baptist isn’t what he claims to be (“Not many evangelicals come out of Cuba,” Trump said). More recently, the real estate developer has escalated his attacks by questioning Cruz’s eligibility to be president, pointing to his Canadian birthplace.

The Texas senator has largely declined to return fire, opting to brush aside questions from reporters or respond with humor. But just days ahead of Thursday’s debate, Cruz has finally started to show his first public signs of impatience.

In an interview with a Massachusetts radio station, Cruz slammed Trump for embodying “New York values,” a dig at the businessman’s past allegiance with Democrats.

He also told reporters in New Hampshire that some of Clinton’s supporters appeared to be “backing Donald Trump.”

“I will say, it is more than a little strange to see Donald relying on as authoritative a liberal, left wing, judicial activist Harvard Law professor who is a huge Hillary supporter,” Cruz said, in reference to Laurence Tribe, who has argued that Cruz is not eligible to be President.

There are signs that Trump’s onslaught is at least resonating with some voters. At a Trump campaign rally over the weekend, one Republican primary voter in the crowd said he was “amazed” to learn that Cruz was born in Canada. And Cruz was bombarded by questions about his ethanol stance during his six-day bus tour last week.

Will anyone come to Cruz’s defense?

Thanks to Trump, Cruz’s eligibility to be president has become a consuming political controversy that’s likely to surface in Thursday’s debate.

And it’s not clear how many of Cruz’s fellow Republicans are willing to defend him.

Cruz is personally disliked by many of his colleagues on Capitol Hill. He has been a thorn in the side of party leaders as he’s earned himself the label of obstructionist during his first few years in the Senate.

Former GOP presidential nominee John McCain — who has not yet endorsed a candidate in the 2016 race — stoked the brewing controversy when he recently said questions about Cruz’s birthplace were “worth looking into.”

Cruz was born in Canada to an American woman and had dual citizenship until recently, when he gave up his Canadian citizenship in 2014. Trump has raised the issue repeatedly on the campaign trail, warning voters that it is certain to create complications and even invite lawsuits.

“Ted is very glib, and he goes out and says, ‘Well, I’m a natural born citizen,'” Trump said on Saturday. “The point is, you’re not.”

So far, at least one of Cruz’s rivals has called it a “non-issue.”

“He is eligible to be president. That is a total non-issue, it really is,” Marco Rubio told CNN’s Dana Bash Tuesday night.

Rubio vs. the governors for the establishment lane

From his work on a bipartisan immigration bill to his poor attendance record in the Senate end even his choice in footwear, the attacks against Rubio have become constant.

Rubio is currently a favorite among the GOP establishment and party leaders. Viewed as one of the more mainstream candidates in the large GOP field with strong head-to-head numbers against Hillary Clinton, the senator is a particularly attractive target for others in the so-called “establishment lane” like Bush, Christie and Kasich.

Those three are all running on their records of governing big states, but by appealing to similar constituencies with a similar overarching message, they have crowded into the same political space trail as Trump, Cruz and Rubio.

That puts a big target first on Rubio’s back.

With Trump and Cruz enjoying sizable leads in Iowa and New Hampshire, as well as nationally, the pressure is growing for someone in the field to unite opposition to them both, and Rubio’s opponents are eager to deny him the opportunity to do just that. Bush’s super PAC has been on a rampage against Rubio, for instance, and the candidate himself has tried to be feistier in his attacks against Rubio and Trump.

A Monmouth University poll released Monday in New Hampshire has Trump leading at 32%, followed by Cruz and Kasich at 14%, Rubio with 12%, Christie at 8% and Bush.

“They realize that only one of them can move onto the next contest after the first two (states),” said Patrick Murray, Monmouth University’s polling director. “They realize you have to winnow down this field of establishment candidates to just one and the only way you’re going to do it at this point is to start taking the other ones out.”

The elephant in the room: Electability

It’s one of Donald Trump’s biggest weaknesses, but so far, his rivals have barely raised it.

Even as the businessman continues to enjoy overwhelming leads in national polls, his ability to compete with Hillary Clinton, the likely Democratic nominee, remains a point of vulnerability.

An NBC News/Wall Street Journal survey last month showed Trump trailing Clinton by 10 points — 40% to 50% — in a hypothetical match-up. The same poll had Cruz also behind Clinton, but by a slimmer margin: 45% to 48%. Rubio, meanwhile, would defeat Clinton in a general election 48% to 45%, the survey showed.

Bernie Sanders, meanwhile, is touting that he would do better than Clinton against top Republicans in Iowa and New Hampshire, citing polls, including the latest NBC/Wall Street Journal/Marist poll.

None of Trump’s GOP rivals has zeroed in on the frontrunner’s relative weakness when it comes to taking on Clinton, the Democratic party’s likely nominee.

“Right now, the personality and the fighter, and somebody that’s going to metaphorically blow up the political system, seems to be more important than how someone may square up in a hypothetical race against Hillary,” said Matt Strawn, former Iowa GOP chairman.

It’s clear the Republican establishment isn’t happy with the way things have gone with Trump. Tuesday, South Carolina Gov. Nikki Haley used the Republican response to the State of the Union address to call out the GOP front-runner, albeit not by name.

“During anxious times, it can be tempting to follow the siren call of the angriest voices,” Haley said.

Remember Ben Carson?

Just a few months ago, there was an unlikely presidential candidate giving Donald Trump a run for his money: Ben Carson.

The retired neurosurgeon’s sudden surge in the polls caught many by surprise. A soft-spoken and low-key candidate who, like Trump, lacked any political experience, Carson seemed out of place in a presidential race, let alone at the top of the polls against rivals like Bush and Rubio.

But unlike Trump, Carson’s moment has turned out to be a blip in the cycle.

Plagued by scrutiny of his biographical stories and struggling to get a handle on foreign policy and national security, Carson began to slip in the polls. His attempts to perform CPR on his flailing campaign — his team underwent a major staff shake-up at the end of the year — have not helped to revive his candidacy.

Carson is not a natural on offense and he does not convey dominance on the debate stage. So it’s difficult to imagine how the candidate might use Thursday’s debate to engineer an electric moment — and ward off being left for dead in the 2016 race. But he has scored well at previous debates.

“Who are his voters? His voters are anti-establishment and social conservatives. These voters have made up their minds that they’re going to stick with Trump and Cruz,” said Murray, the Monmouth University pollster. “So Ben Carson’s day has come and gone.”

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