Pressure is on top-tier candidates to break out from the crowded field.

Jeb Bush

The spark that his supporters had hoped to see in the first debate wasn't there. Since then, he's been taking mild pokes at Trump, showcased a sense of humor on Stephen Colbert's late-night debut and rolled out some serious policy proposals. But he needs to stop his slide. His big warchest, which he's begun to deploy to buy TV ads, will keep him in the mix.

Ben Carson

He's been soaring since the Aug. 6 debate, where his attention-getting calm persona helped boost his fundraising. In some national polls, he's running a solid second to Trump. In Iowa, he's tied or just behind the frontrunner.

Chris Christie

The New Jersey governor joked that he's going to "go nuclear" at Wednesday's debate if he doesn't get his fair share of questions. Once considered a formidable presidential prospect, he's now pulling single-digit support in national polls. He's trying to boost his campaign with TV ads that focus on his record and recently noted that the first votes are still months away.

Ted Cruz

He's been echoing some of Trump's views and shared a stage with him last week at a rally opposing the Iran nuclear deal. There's a sense that he's positioning himself to pick up Trump's support if and when the frontrunner's campaign unravels. But what if it doesn't?

Carly Fiorina

She capitalized on strong reviews of her performance in the Aug. 6 second-tier debate, parlaying that exposure into a position in the middle of the GOP pack and a spot in the prime-time debate on Wednesday. An encore could propel the GOP race's only woman even higher.

Mike Huckabee

There he was last week, standing next to Kim Davis, the Kentucky county clerk who had been jailed for refusing to issue marriage licenses to same-sex couples. The move epitomized his campaign's focus on Christian conservatives — who are a powerful bloc among Iowa Republicans.

John Kasich

He was considered a long shot before the first debate, but was praised for parrying Trump and showcasing his conservative message. His showing won him more donations, a second look from many voters and higher poll ratings. As a result, expectations for him are high in this week's debate.

Rand Paul

He didn't really distinguish himself in the first debate and since then has largely faded from view. He has said he'll be more assertive this time. The race's strongest libertarian is trying to regain his footing by challenging his party's leadership on such issues as funding for Planned Parenthood.

Marco Rubio

He's still young, still energetic, still has an appealing personal story and is still admired by conservative voters. But he's stuck in the middle of the pack with dwindling poll numbers. If he can steal — or at least share — the spotlight on Wednesday, it might not be too late for a surge.

Donald Trump

His outrageous comments (last week he insulted Carly Fiorina's appearance) and unabashed lack of knowledge on foreign policy and other matters haven't made a dent in his popularity. Debate moderator Megyn Kelly was tougher on him in the first debate than his opponents were; will they rough him up this time?

Scott Walker

Once considered an automatic top contender because of his experience and conservative credentials, his poll ratings have sunk to single digits and his aides are talking about a campaign "reboot." He needs to find a way to prove that he's still viable before voters write him off as bland.