Washington: Republican frontrunner Donald Trump`s taunt Thursday of "sad" rival Jeb Bush -- "he was supposed to win, and he just doesn`t have the energy" -- speaks volumes about the billionaire`s propensity to go for the jugular.
But more importantly it spotlights challenges facing the man once seen as his party`s most electable presidential candidate in 2016.
The Bush campaign has shown puzzling lack of traction against brash real-estate mogul Trump as he seeks to break out from the rest of the crowded field, reinforcing American voter skepticism about Jeb following his father`s and brother`s footsteps into the White House.
He has slid steadily in the polls, from national frontrunner in mid-July down to single-digit support today.
In a video clip circulated on the Internet by pro-Democrat groups, an attendee at a Bush town hall is seen napping, re-enforcing Trump`s biting charge last month that Bush just does not excite voters.
And there is the baggage of brother George W. Bush`s Iraq, along with Jeb`s fumbling of his own message about the invasion.
Some influential conservative pundits praise Jeb Bush as well-intentioned and good-hearted, but they knock him as a has-been who fails to rally a base desperate for new blood.
"Not since 1980 have we won without" a Bush on the ballot, Erick Erickson wrote on Red State, a popular blog that he edits.
"But I`m not prepared to say the party of Lincoln is dependent on the Dynasty of Bush."Polls suggest Republican voters have apprehensions about Bush.
In Iowa, which holds the first nominating contest in February, Bush is in sixth place, according to a RealClearPolitics poll average, behind Trump and two other political outsiders, neurologist Ben Carson and business executive Carly Fiorina.
In New Hampshire he is third, down from first in mid-July.
These are not the numbers or optics of a White House winner.
"I don`t think there`s any doubt that Bush understands he has to change the narrative of the campaign," G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin and Marshall College, told AFP.
"People expected a lot more."
Political outsiders like Trump and Carson appeal to the anti-Washington, populist sentiment shared by many conservatives.
"Bush is trapped into being the quintessential establishment politician" during a period when the establishment is intensely disliked and distrusted, Madonna said.
"That`s a tough dynamic for him."
With Trump dominating cable television coverage, Bush has struggled to advance his message, including a compassionate immigration system and a politics of inclusion that he and other Republicans insist is vital to winning the White House.
One reason Bush may not be resonating like Trump, argued veteran political observer Larry Sabato of the University of Virginia, is that Bush is among the status-quo politicians who are seen as "human tape recorders," who regurgitate consultant-approved soundbites.
"Whatever else you think of him, Trump is no automaton," Sabato wrote in his "Crystal Ball" column.
"We can`t take our eyes off him because we`re not at all sure what he`s going to say next."
Bush has spent much of the past week punching back against the frontrunner, insisting Trump was more Democrat than Republican and highlighting his previous support of abortion rights and tax hikes for the wealthy.
After Trump said he thought Bush -- who sometimes answers questions in Spanish while on the campaign trail -- should be "speaking English," Bush blasted "The Donald" with some of his most forceful comments to date.
"Trump is trying to insult his way into the presidency," Bush told ABC News.
"It`s hurtful for a lot of people and Mr. Trump knows this," he added. "He`s appealing to people`s angst and their fears rather than their higher hopes.
"We need to be unified."
Between Trump and Bush, recent history is on Bush`s side.
There have been many political insurgents in recent decades aiming to upset the status quo, only to see Republicans coalesce around an establishment candidate like Mitt Romney, John McCain -- or George W. Bush.
But this Republican nominations process, packed with 17 candidates and fuelled by the rise of the anti-politician, is the "least predictable" in modern times, Madonna said.
Conventional wisdom suggests the Trump phenomenon fades within months, once the field narrows and rivals convince voters that he is insufficiently conservative.
"Question is," pondered Madonna, "does conventional wisdom work this year?"