In the first Republican presidential debate, most candidates took a hands-off approach to Cand hoped the brash billionaire would hurt himself.

Instead, he only got stronger. Trump’s unexpected durability has led some of his rivals to shift their strategy for Thurday’s second debate.

Now their goal is to engage Trump, without inflicting any damage on their own campaigns.

The change reflects an evolution in the way Trump is viewed within the Republican Party. No longer dismissed as a summer fling for frustrated voters, Republicans increasingly see Trump as a candidate who could remain atop the field for months and win some early state contests.

"He’s in complete, total control of the political battle space," said Steve Schmidt, a top strategist for Sen. John McCain’s 2008 presidential campaign.

Trump will be standing at center stage when the 11 candidates face off at the CNN-sponsored debate at the Reagan Presidential Library in Simi Valley, California. The lineup is the same as for last month’s opening debate, with one notable addition: former Hewlett-Packard chief executive Carly Fiorina, the only woman in the Republican field.

It’s believed to be the largest debate field in modern political history, underscoring just how jumbled the fight for the Republican nomination remains with five months to go before the leadoff Iowa caucuses.

Trump heads into the second debate facing a fresh challenge from Ben Carson, a soft-spoken retired neurosurgeon who has been climbing in recent polls. With his candidacy on the rise, Carson is likely to face heightened scrutiny from debate moderators.

But the biggest contrast in the Republican field and the one that typifies the broader battle within the party continues between Trump and Jeb Bush, the former Florida governor.

Bush has become one of Trump’s favorite targets and has been visibly irritated by his jabs, particularly the reality TV star’s depiction of him as a "low energy" candidate.

After all but ignoring Trump in the first debate, Bush has gone after the Republican front-runner more aggressively while campaigning.

People familiar with Bush’s debate plans say he’ll actively look for spots to target Trump, particularly for the real estate mogul’s uneven record as a conservative, but still wants to preserve space to pitch himself as an optimistic alternative.