But the fear remains in some of the campaigns that Mr. Trump will target their candidates in the lead up to the debate — or at the event itself.

Many national Republican officials are increasingly resigned to Mr. Trump’s looming presence. At a meeting of the Republican Governors Association this week in Aspen, Colo., donors and operatives mused about how to prevent him from hijacking the debate.

One idea that came up was to urge three leading candidates — Jeb Bush, the former Florida governor; Mr. Walker; and Senator Marco Rubio of Florida — to band together and state that they would not participate in any debate in which Mr. Trump was present, using his refusal to rule out a third-party bid as a pretext for taking such a hard line. The thinking, according to a Republican involved in the conversations, was that the lesser-funded prospects who have been eclipsed by Mr. Trump would follow suit, and the TV networks airing the debates would be forced to bar Mr. Trump in order to have a full complement of candidates.

But none of the campaigns have shown any appetite for such solidarity, for reasons ranging from their strategic interests and not wanting to make Mr. Trump a martyr, to fear of making an enemy of Fox News, the preferred cable network of conservatives and the host of the first debate.

And officials at the Republican National Committee, which condemned Mr. Trump’s comments about Mr. McCain over the weekend, do not believe Fox would consider dropping Mr. Trump, regardless of any potential statement from them or any of the campaigns.

All this has led the candidates and party leaders to take a this-too-shall-pass approach, hoping that the media-fueled Trump boomlet will fade like most summer flings as Republicans begin to take a longer look at their options later this year.