BOULDER, CO — (CNN) The Republican presidential campaign is entering a decidedly more combative phase as candidates prepare to take the debate stage on Wednesday night.

The GOP field is seeing a shake-up after remaining static for weeks. Long-time front-runner Donald Trump no longer appears invincible, thanks to the climb of Ben Carson. Jeb Bush has hit hard times, but promises to show a more muscular side. Carly Fiorina failed to turn her strong debate performances into winning poll numbers. And since the last debate in mid-September, Wisconsin Gov. Scott Walker has left the race.

Here are six things to watch in the CNBC debate:

Fight night: Trump vs. Carson

From his perch at the top of the Republican field, Trump had largely declined to go after Ben Carson, training his fire instead on other more seasoned politicians like Bush, Marco Rubio and Rand Paul.

Referring to Carson as a “good” person who he admires personally, Trump has even fueled speculation the retired neurosurgeon could be on his vice presidential short list.

But those days of playing nice are over.

An abrupt shift in tone came after a series of consecutive polls showed Carson had overtaken Trump in Iowa, relegating the businessman to second place. Then, on Tuesday, Trump even lost his first place spot in a national poll, trailing Carson 26% to 22%, according to Tuesday’s CBS/New York Times poll.

If the last week offers any indication, Trump is expected to throw multiple punches at Carson on the debate stage. The attacks could get personal. The real estate mogul appeared to suggest over the weekend that Carson’s religious affiliation — he’s a Seventh-day Adventist — was extreme.

But it will take a whole lot more to get Carson firing back. Known for his reserved and calm demeanor, the candidate has repeatedly said he has no interest in becoming an attack dog.

“Ben has said he’s going to stay who he is and he wouldn’t want to get elected being somebody else,” said Ryan Rhodes, Carson’s Iowa state director. “He does not need to tear someone else down to build himself up.”

Jeb Bush, no longer center stage

When Bush steps onto the stage Wednesday night, there will be visual confirmation of his recent struggles. Rather than take the podium immediately next to Trump, Bush will be two spots removed from the GOP front-runner for the first time.

The former Florida governor’s national poll numbers seem permanently stuck in the single digits behind political novices like Trump and Carson. Last week, the campaign announced new cost-cutting measures, followed by a weekend Bush family confab in Houston.

“He needs to have a performance that stands out from the crowd. He won’t be center stage this time so he need to, in words and actions, figure out how not to fade into the wings,” said Katie Packer, deputy campaign manager for Mitt Romney’s 2012 campaign. (Packer is neutral in the race but is a founding partner of WWP Strategies, which works for Rubio in Michigan.)

Bush’s advisers are suggesting Bush may try to do just that on Wednesday, by showing off his more fiery and loose side. The campaign is “tearing up the script” and preparing to simply “let Jeb be Jeb,” sources told CNN’s Jamie Gangel.

Carly Fiorina: Third debate’s the charm?

There’s no question that Carly Fiorina is the star of the Republican debate team.

What’s less clear is whether she can translate the momentum from her prime-time performances into a lasting boost for her candidacy.

The former Hewlett-Packard CEO used a breakout performance in the last CNN GOP debate to propel herself from a little-known, first-time presidential candidate to a top-tier contender. But that surge in the national polls has all but vanished; she fell from 15% in September to 4% in last week’s CNN/ORC survey.

Her campaign has pushed back on any suggestion that Fiorina may be going from one breakout moment to the next, and instead emphasized that she’s focused on winning over voters at a slow and steady pace.

“I’m glad people liked her debate performance and liked her message, but one debate does not win the thing,” deputy campaign manager Sarah Isgur Flores said.

Debt ceiling: To raise or not to raise

In a debate that’s promising to focus on jobs and the economy, one issue could divide the Republican candidates on the stage Wednesday: the debt limit.

Raising the debt ceiling is a Congress-approved maneuver that essentially allows the U.S. government to carry more debt in order to allow the Treasury Department to continue paying its bills. The issue has become highly divisive among Republicans in recent years: Fiscal conservatives argue they would rather let the country default on its financial obligations than increase national spending.

This week, congressional leaders and the White House reached a two-year budget deal that would raise it through early 2017.

“Raising the debt ceiling is politically explosive on the right. Some candidates pander and say they never will raise it,” said Douglas Holtz-Eakin, president of the American Action Forum and economic adviser to John McCain in 2008. “Others will invoke something like the Boehner rule and argue that spending cuts will make it worth it. But the real test is who will say ‘yes, we have to raise it’.”

Crunch time for the bottom-tiers

Of the 10 Republicans participating in the prime-time debate, these four rank at the bottom: Mike Huckabee, Chris Christie, John Kasich and Rand Paul. And they’ve been there most of the race.

According to CNBC’s average of recent national surveys, each of those Republicans poll around 3%. With the Iowa caucuses now just three months away, these candidates are under pressure not only to boost their rankings, but also to persuade donors to keep funding their bottom-tier campaigns.

Christie’s campaign raised just over $4 million in the third quarter while Kasich raised around $4.4 million since launching his campaign. That’s not much more than the $3.9 million Trump’s campaign raised without actually trying. Meanwhile, Rand Paul raised $2.5 million last quarter, while Mike Huckabee pulled in $1.2 million.

Will there be much of a debate?

Every candidate faces a shared challenge Wednesday night: persuading viewers not to change the channel.

After coming under fire from candidates like Trump and Carson, CNBC agreed to keep Wednesday’s prime time debate to just two hours, commercials included. The network also agreed to allow opening and closing statements for each candidate, which means even less time for the White House hopefuls to tackle questions, go after rivals and make a splash.

The first two GOP debates were record-setting blockbusters, pulling in an unprecedented 24 million and 23 million viewers each.

Not only are those numbers becoming increasingly hard to beat as the novelty of the debates wears off, but the debate also happens to collide with Game 2 of the World Series between the New York Mets and the Kansas City Royals.

The-CNN-Wire ™ & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved. (PHOTO: CNN)