A lot on the line at Tuesday’s GOP presidential debate

The undercard

The undercard begins at 7 p.m. Participants: New Jersey Gov. Chris Christie, former Arkansas Gov. Mike Huckabee, Louisiana Gov. Bobby Jindal and former Pennsylvania U.S. Sen. Rick Santorum.

Two candidates who participated in the three previous undercard debates, former New York Gov. George Pataki and South Carolina U.S. Sen. Lindsey Graham, did not qualify this time, nor did former Virginia Gov. Jim Gilmore.

Polling

Qualification to participate in the prime-time debate required a candidate to average at least 2.5 percent in the four most recent national polls. The bar was set at 1 percent for the undercard debate.

The averages were passed on polls conducted by Fox News, Investor’s Business Daily/TIPP, Quinnipiac University, and The Wall Street Journal/NBC News.

A restive Republican field meets again Tuesday with more on the line than ever before.

Billionaire Donald Trump hopes to reignite his campaign's momentum after falling in the polls from a September high, while neurosurgeon Ben Carson, joining Trump at the top of the national polls, hones his more combative side after new questions were raised about his celebrated background.

Florida U.S. Sen. Marco Rubio is trying to quell concerns about his personal finances. And ex-Florida Gov. Jeb Bush, long the assumed establishment favorite, faces growing criticism after his flat performances in the previous three debates.

At least they’ll be on the main stage. New Jersey Gov. Chris Christie and former Arkansas Gov. Mike Huckabee are both bristling after they were relegated to the undercard — an earlier debate along with two lesser-known rivals — due to dismal poll standings.

It’s the media, though, that may be under the most scrutiny for Tuesday night’s showdown, the nation’s newest version of must-see reality TV.

The campaigns and candidates erupted at CNBC’s moderators after last month’s debate over what they deemed to be slanted “gotcha” questions. This time, it’s the Fox Business Network in the pilot’s seat as millions of Americans tune in for what hosts promise to be a “real debate.”

That’s what Cassandra Briones, a mother of four in Waukesha, wanted to hear.

“I hope that they focus on important issues and not just entertainment,” she said as her twin 5-year-olds played around her at a charter school in this town on Milwaukee’s outskirts. “I’m ready for them to talk about what matters.”

Twists and turns

The race has taken more twists than a neurotic pretzel since that rocky debate in Colorado, and the candidates all have plenty to gain — and as much to lose — from the fourth GOP showdown of the year.

Each one has been a ratings bonanza, largely driven by Trump’s say-anything bombast, and each also holds game-changing political potential for candidates trying to stand out in such a crowded field.

Bush may have the most to lose. He tried to reboot his campaign after a frustrating fall with a "Jeb Can Fix It" tour that invokes his eight years in Florida's top job. But he remains stuck in the single digits in national polls despite financial firepower that's helped raise more than $100 million for his White House bid.

He courted Wisconsin Gov. Scott Walker on Monday, praising his one-time White House rival’s embrace of charter schools. Walker, once another favorite of establishment Republicans, dropped out of the race in September and has yet to endorse another candidate.

Suddenly surging in the polls, Carson has a new target on his back with reports questioning his claim that West Point offered him a scholarship and other key elements of his biography. He’s accused the media of embarking on a “witch hunt” to disparage the personal narrative of redemption that’s helped him draw evangelical voters.

"I'm getting special scrutiny. Because, you know, there are a lot of people very threatened," Carson said Sunday on CBS "Face the Nation," adding: "And the whole point is to distract the populace, distract me. You know, if you've got a real scandal, if you've got something that's really important, let's talk about that."

Rubio, another candidate on the rise, will face questions about his use of a Republican Party of Florida credit card in 2005 and 2006 when he was a young Florida lawmaker. Credit card statements released over the weekend showed about $7,200 in personal purchases, which Rubio's aides said he later repaid, and raise questions about his money management skills.

A tighter debate

The debate will also feature another first for the year: A smaller field.

Host Fox Business Network cut Christie and Huckabee from the prime-time debate after polls showed them below the 2.5 percent threshold, a demotion that could haunt their campaigns.

Christie, who was beginning to show traction in some national polls, tried to portray it as a minor setback. But he’s also railed against poll-centric reporters who ignore “what’s happening in New Hampshire and Iowa,” the home of the first presidential contests in 2016.

That's a common refrain from the Republican field and the reason why the media may have the most to prove Tuesday. Republicans, and plenty of other critics, slammed CNBC's handling of the last debate as a derisive and hostile affair. Many of the campaigns cobbled together a series of demands for future debates, though they face an uncertain end.

The Fox Business moderators told reporters they are preparing to press the candidates with skeptical, but not snarky, questions. The candidates will be poised to hit back hard at any hint of media bias.

Then there’s the reaction of President Barack Obama, who offered his own take on the line of questioning from that October debate.

“If you can’t handle those guys,” he said of the CNBC moderators, “I don’t think the Chinese and the Russians are going to be too worried about you.”