But it isn’t just Bush’s energy level. Trump has an instinct for finding the weak spots in his rivals’ records that will get under their skin. Trump doesn’t always go for full factual accuracy, but puts a personalized spin on vulnerabilities and delivery. Among his targets so far: Wisconsin Governor Scott Walker’s economic record, Fiorina’s business background, Florida Senator Marco Rubio’s immigration stance, Bush’s ties to lobbying and corporate interests, and, of course, the poll standing of everyone else. And knowing the voters appreciate that he is a hard-nosed negotiator, Trump cleverly damns all of his rivals by calling them “nice,” by which he means they are not up to the job of dealing with China, Japan, and Mexico.

Of late, Trump has glaringly overstepped on occasion, for instance deeming the world-renowned neurosurgeon Carson an “OK doctor.” It will be interesting to observe if Trump uses more circumspection going forward, employs surrogates, or pumps some of his private cash into negative advertisements. So far, he hasn’t needed any of that.

After the Summer of Trump, with most of his opponents yipping toothlessly or running scared, some of the candidates are donning Trump-sized gloves. Louisiana Governor Bobby Jindal last week launched an extended tirade, calling Trump shallow, a narcissist, an egomaniac, and a carnival act, later adding insult to insult with the remark that it looks like “he’s got a squirrel on his head.” Fiorina has dismissed Trump’s latest jabs with smirking scorn (and, her supporters, claim, has laid the groundwork to succeed in taking on Trump where her male counterparts have abjectly failed). Perry departed the race with an exhortation to his fellow Republicans not to “nominate a candidate whose rhetoric speaks louder than his record.”

But part of the problem for the candidates who want to try to kill Trump is that they have other stuff to do, stuff that Trump has no worries about: getting better known; raising money; currying favor with the press; seeking endorsements; demonstrating he or she is ready to be commander-in-chief. As Bush and others have seen, taking on Trump creates a media environment where the only message you can get out is about your battle with Trump. Based on what’s happened so far, that is at best a severe distraction, and in most instances it has turned out a whole lot worse. But the alternative is to continue to let Trump totally dominate the race, which has become the biggest frustration of nearly all of the candidates and campaigns.

Perhaps we will have more data this Wednesday night when the top 11 Republican candidates come together on stage in California for the second debate. Every campaign is considering the risks and rewards of challenging Trump and won’t back away from a fight initiated by the questioners or Trump himself, but several senior advisers to other candidates have told me they remain wary of instigating such a bold move. On a crowded stage with limited time, taking on a seasoned TV star still seems foolhardy when the campaign cycle is swinging into autumn.

Along with the unknown of “who” will go after Trump comes the question of “how” they will try to stop him eventually. Discussions with numerous highly interested Republican and conservative strategists over the last two weeks yield very little consensus. No campaign wants to spend TV ad money now to go after him. The Club for Growth, a Washington advocacy group, is one entity that is willing to spend money to try to derail the front-runner. The group has long been at odds with Trump, and announced at a press conference on Tuesday a $1 million ad buy going after the billionaire that will air on broadcast, cable and satellite television in Iowa and on the web. The two 30-second ads attack Trump on some of his "very liberal" policies and for his support on "eminent domain abuse," but it surely isn't enough to bring him down and the group is suspect in some eyes (they recently hit him up for a $1 million contribution, undermining their credibility). The Koch brothers, the Chamber of Commerce, and other names are bandied about, but there is no sign any of them plan to take on Trump imminently.

Then there’s the question of what message “frame” (to use the politics term of art) to deploy against Trump. Part of the difficulty here is that most conventional strategies have been tried already (and featured regularly in the news media) and have had no discernible impact on his rise. Several of the other Republican campaigns have robust opposition research operations in place, and along with their lobbyist, Wall Street, and congressional allies, are looking for additional background on Trump to forge a clear line of attack.

The current thinking on possible frames falls into four distinct but overlapping stratagems:

1. Trump can’t be trusted because he is an egomaniac with a bad character (his business dealings; his bankruptcies; his two divorces; his insulting statements about women; his casinos; his compulsion to name things after himself; his hazy answers about his own faith; and his aggressive use of eminent domain).

2. Trump is a liberal and unprincipled (past support for single payer, the Clintons, House Democratic Leader Nancy Pelosi, and abortion).

3. Trump is not close to being fit to be a serious president or commander-in-chief (his recent answers on foreign policy; his lack of specifics on almost everything).

4. Trump is a politician, not a businessman/outsider (his long-time retention of Beltway lobbyists; his self-serving campaign contributions; his courtship of local, state, and national officeholders).

All of these frames have some potential, according to those who wish to end Trump’s domination of the Republican nomination process. But picking one (or two), and driving the message to success is as difficult as it is vital to his foes. There is by no means consensus on which of these frames will work or how to flesh them out. One school of thought, according to my reporting, is to go with an amalgamation of “deals and females,” on the theory that those are Trump's twin vices and the opposition could use those issues to at least pry away some parts of his current coalition and bring his poll numbers down closer to the pack. The hope is that should Trump's numbers go lower, he will get rattled, his aura of invincibility will dissipate, and his mojo will be disrupted.