Donald J. Trump’s surge in the polls has been met with barely concealed delight by Jeb Bush and his supporters. Mr. Trump’s bombastic ways have simultaneously made it all but impossible for those vying to be the alternative to Mr. Bush to emerge, and easier for Mr. Bush, the former Florida governor, to position himself as the serious and thoughtful alternative to a candidate who has upended the early nominating process.

With little indication that his support is slipping and the promise of the center stage at Thursday’s debate, Mr. Trump has essentially frozen the rest of the field.

“The longer it goes, the greater the panic is going to build,” said Alex Castellanos, a longtime Republican strategist. “And that means you may not have the luxury to flirt with an undeveloped, budding candidate. Trump has set the Republican Party on fire, and if you’re going to put that fire out you don’t have time to waste. You’re going to have to grab the biggest blanket you got and throw it, and right now that’s Jeb.”