WASHINGTON — Most candidates took a hands-off approach to Donald Trump in the first Republican Presidential debate and 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 the Sept. 15 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 has been a thorn in the side of the Republican establishment, ratcheting up anti-immigration rhetoric that they fear will damage the party’s image among Latino voters and offering support on occasion for tax hikes and universal health care, issues that have long been considered taboo among conservatives.

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.

Wisconsin Gov. Scott Walker has also forecast a more aggressive stance at the event. Once a favorite to win the Iowa caucuses, Walker’s standing has plummeted after an unremarkable first debate and a series of other summer stumbles.

Even if candidates follow through on their pledges to be more aggressive, there are warning signs about that approach.

Trump has so far been immune to criticism of his lack of specific policy proposals, personal attacks on women and immigrants, and his commitment to conservatism.

And with voters increasingly drawn to anti-establishment candidates, it’s unclear whether attacks from those with long political resumes will be effective.

Fiorina is also a political outsider, having spent most of her career in business. She’s also shown no fear in taking on Trump, including after he was recently quoted insulting her appearance. She’s also thrilled Republican audiences with her sharp criticism of Democratic front-runner Hillary Rodham Clinton.

Fiorina didn’t have enough support in the national polls used to select debate participants to get on the main stage in August, but she was a standout in the earlier undercard debate.

After an aggressive lobbying effort by her campaign, CNN broadened its participation requirements to allow her on the main stage.

Carson, too, appears to be benefiting from the public’s anti-Washington mood. As his standing has risen, he’s waffled in his approach toward Trump, first questioning the businessman’s faith, then apologizing for doing so.

Campaign manager Barry Bennett said Carson has no plans to take on Trump in the debate. Florida Sen. Marco Rubio also doesn’t plan to take on Trump.

Also on stage will be Ohio Gov. John Kasich, who is well-liked among more moderate, business-minded Republicans, and former Arkansas Gov. Mike Huckabee, a favorite of social conservatives.

They’ll be joined by Kentucky Sen. Rand Paul, New Jersey Gov. Chris Christie, and Texas Sen. Ted Cruz, the sole contender to align himself with Trump in hopes of winning over his supporters should Trump stumble.

Four candidates lagging behind in national polls did not qualify for the main event and will be relegated to an earlier debate: Louisiana Gov. Bobby Jindal, South Carolina Sen. Lindsey Graham, former Pennsylvania Sen. Rick Santorum and former New York Gov. George Pataki.

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By Julie Pace, AP White House Correspondent. AP writers Thomas Beaumont in Burbank, California; Scott Bauer in Madison, Wisconsin; and Steve Peoples in Los Angeles