WASHINGTON - The so-called "undercard" of the fourth debate among Republican candidates for U.S. president has begun, while the top eight participants wait for the main event to be held in the north-central city of Milwaukee, Wisconsin.

New Jersey Governor Chris Christie and former Arkansas governor Mike Huckabee were demoted to the "undercard" debate," while Senator Lindsey Graham of South Carolina and former New York governor George Pataki failed to qualify for either contest.

Christie, Huckabee, former Pennsylvania senator Rick Santorum and Louisiana Governor Bobby Jindal all took the opportunity early in their pre-debate to promote plans they claim will make the U.S. tax code fairer. Huckabee even proposed getting rid of the internal Revenue Service, which is responsible for tax collection and tax law enforcement.

Debate sponsor Fox Business promised this debate, focused on the economy, would be an improvement over October’s debate, which set off a round of criticism from the candidates and conservatives, who said it lacked substance.

This one will be the “real debate about our economy and future,” said Fox in a commercial for the debate, indicating that the cable network would aim to focus on the issues. But the run-up to the debate was about anything but.

For starters, there was speculation about the smaller number of participants. Will it make a difference? Only the top eight candidates will participate in the main debate, based on a minimum standing of at least 2.5 percent in the latest polls.

Republican presidential candidate Ben Carson speak
Republican presidential candidate Ben Carson speaks to reporters during a news conference Nov. 6, 2015, in Palm Beach Gardens, Florida.

? Carson under scrutiny

A new McClatchy/Marist poll, released just before the debate, showed candidates Ben Carson and Donald Trump still running neck-and-neck. Carson had 24 percent and Trump had 23 percent. Florida Senator Marco Rubio was a distant third with 12 percent, while Texas Senator Ted Cruz and former Florida governor Jeb Bush were tied for fourth at 8 percent.

Tuesday's debate is sponsored by the Fox Business Network and will focus on the economy. But much of the spotlight is likely to be on the two top contenders at the moment, Donald Trump and Ben Carson. Carson has been on the defensive in recent days after numerous questions about incidents in his past including a claim that he once was offered a full scholarship to the U.S. Military Academy at West Point, New York.

Carson can expect more questions about his past during the debate, but it also appears he will be ready with some feisty comebacks. At a Friday news conference in Florida, Carson said he wanted the focus on unfair treatment by the media." All of you guys trying to pile on is actually going to help me because when I go out to these book signings and I see these thousands of people they say, Do not let the media get you down.'"

GOP 2016 SNL Trump
Protesters opposed Republican presidential candidate Donald Trump demonstrate in front of NBC Studios Wednesday, Nov. 4. Trump hosted NBC’s "Saturday Night Live” on Nov. 7.

? Trump center-stage again 

Donald Trump, who shares the lead with Carson, was in the spotlight once again as he hosted NBC's "Saturday Night Live," even as anti-Trump protesters made their presence felt on the streets outside. Although one of the highest-rated SNL shows in recent years, a lot of critics thought the Trump-hosted show was dull.

Will he be dull in the debate? Observers say he tends to fall back when policy matters are discussed in detail, rasing questions about his seriousness as a candidate.

The latest polls show Trump and Carson bunched at the top of the crowded Republican field with Florida Senator Marco Rubio and Texas Senator Ted Cruz also picking up steam. Rubio was considered a winner in last month's Republican debate, although most of the candidates complained that the questions from the CNBC moderators were biased and unfair.

Rubio may well be under fire for his use of a Florida Republican Party credit card during his time in state office. Rubio released credit card records this weekend to head off criticism.

Republican presidential candidate, former Florida
Republican presidential candidate, former Florida Gov. Jeb Bush, gestures as he speaks to supporters during a rally Nov. 2, 2015, in Tampa, Florida.

The Bush reboot

Tuesday's debate also will be a test for Jeb Bush, who has slipped in recent polls and remains focused on a reboot of his campaign, now running under the theme of "Jeb Can Fix It. "I will not trade in an optimistic outlook to put on the cloak of an angry agitator, and I will not make anyone feel small so I can fee big," Bush said during his relaunch in Florida.

A new book about the candid opinions of his father, former president George H.W. Bush, has become a distraction and a reminder of the unpopular presidency of his brother, George W. Bush, said analyst Norm Ornstein of the American Enterprise Institute.

"[Jeb] Bush has the dilemma right now that he is trapped in a Bush body and simply cannot and will not gain the kind of distance from the policies of his brother and father that is required if you are going to get back to a dominant position," Ornstein said.