Can Donald Trump stay on top, hold off surging Ben Carson at key CNN debate?

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SIMI VALLEY, California — Donald Trump is barreling into tonight’s CNN debate with more than a dozen other candidates aiming to take him on and break through the cluttered field.

The billionaire businessman has dominated the GOP primary race all summer despite controversial comments about immigrants and women that would have doomed other candidates. But when he walks on stage tonight at the Ronald Reagan Presidential Library, he’s likely to face sustained attacks from multiple fronts and will have to prove he can handle the pressure.

Going into the night, Trump says he might try a different approach.

“I could tone it down a little bit when pressed,” he told the Christian Broadcasting Network in an interview airing Wednesday. “You need a person of tremendous strength but I think I can tone it down a little bit and I’ll try.”

The debate comes at a crucial point in the GOP primary with tensions running high and establishment candidates struggling against their outsider counterparts.

The candidates started arriving at the debate hall early afternoon to “walk through” the evening and get their bearings on the stage. Wisconsin Gov. Scott Walker, Florida Sen. Marco Rubio and former Arkansas Gov. Mike Huckabee and former Florida Gov. Jeb Bush were among the first to do their practice runs. Each took their respective spots behind the podiums, familiarizing themselves with the position of the cameras and moderators, as well as the format of the debate.

Trump is suddenly facing stiff competition from retired neurosurgeon Ben Carson, who is surging in the polls. The two, who are most effectively harnessing the desire among many primary voters for an outsider candidate, have sparred over the past week about their faith.

But Trump may face the most intense attacks from Bush and Carly Fiorina.

Fiorina, whose breakout performance at last month’s debate helped her land a spot in tonight’s prime-time showdown, is intent on upstaging Trump. Trump has attacked Fiorina’s business record as the CEO of Hewlett-Packard while she has slammed Trump for being light on substance.

Their sparring intensified over the past week after Rolling Stone published an interview with Trump in which he dismissed Fiorina by saying, “Look at that face! Would anyone vote for that?”

Fiorina shot back: “I am proud of every year and every wrinkle.”

Bush may have the most at stake in the debate. Despite his $100 million bank account and record as a two-term governor running one of the country’s largest — and most complex — states, Bush is tumbling in the polls.

After a lackluster performance last month, he’ll have to prove that he can turn his troubled campaign around. He’ll do that, in part, by taking on Trump directly after spending much of the summer ignoring his presence.

Bush is already taking a more aggressive stance. In recent interviews and posts on social media, the former Florida governor has repeatedly questioned Trump’s conservative bona fides, slamming the businessman on issues like immigration, healthcare and taxes.

A new web video released the week from Right to Rise, the pro-Bush, super PAC could preview the type of message he might offer tonight. The video labeled Trump as a candidate “in a very dark place” before presenting Bush as someone who is choosing a “brighter path.”