Republican 2016 presidential candidate Donald Trump, left, talks with fellow candidate and former governor of Florida Jeb Bush during a break at the first official Republican presidential debate Cleveland, Ohio, August 6, 2015. REUTERS/Brian Snyder During the first Republican debate earlier this month, Fox News host Megyn Kelly pointedly asked former Florida Gov. Jeb Bush (R) about comments Bush allegedly made calling Donald Trump an "a------."

Bush shook his head.

"It's not true, but I have said that Mr. Trump's language is divisive," Bush said.

The response yielded a moment of civility between two top Republican presidential candidates. Trump subsequently praised the former governor, calling him a "true gentleman."

"First of all, Jeb, I am very happy that you denied that, and I appreciate that very much," Trump responded. "I mean, he's a true gentleman."

But the moment of detente has passed.

As Trump has continued to dominate the early polls, Bush is abruptly shifting strategy, going after Trump head-on and repeatedly launching some of his toughest shots yet at Trump over the past few days.

It's a risky strategy — dueling with Trump has only plunged candidates like Sen. Rand Paul (R-Kentucky), Sen. Lindsey Graham (R-South Carolina), and former Texas Gov. Rick Perry (R).

Some Republican strategists consider it necessary for Bush, the candidate who has yet to dominate in the way many thought he could and would, to light a spark in his campaign. He trails Trump by double digits in national polls and in New Hampshire. And in the Granite State, he's getting squeezed by candidates like Ohio Gov. John Kasich (R).

But others question what's in it for Bush.

"I suspect Jeb is going after Trump for two reasons: 1. Trump has gotten under Jeb’s skin; 2. Going after Trump gives Jeb a chance to appeal to conservatives and remind them of his conservative record," said Matt Mackowiak, the founder and president of the Potomac Strategy Group.

"But Jeb isn’t losing any voters to Trump, so I’m not sure what can be gained."

Former Florida Gov. Jeb Bush at the Defending the American Dream summit hosted by Americans for Prosperity, August 21, 2015. AP

The first fireworks went off in New Hampshire this week, when Bush and Trump held dueling events. Perhaps it was only a coincidence, but Trump's campaign announced his New Hampshire event a day after Bush's campaign released word of its Granite State swing.

Trump continued his line of attack against Bush both at the event and in backstage remarks with reporters. Miles away, Bush went directly after Trump.

"There's a big difference between Donald Trump and me," Bush said in response to a question about Trump during a town-hall event. "I'm a proven conservative with a record. He isn’t."

Bush highlighted Trump's past as a "tax-hiking Democrat," noting that he had supported tax hikes on people who have more than $10 million in assets, as well as a single-payer healthcare system.

"He’s been a Democrat longer than a Republican," Bush said.

His campaign almost immediately cut the video, blasting it out to reporters and tweeting the quote.

Bush continued to draw a contrast between himself and Trump on Thursday. On Friday, Bush took another shot at the real-estate mogul — this time, at his immigration plan unveiled last weekend.

And late Friday night, as Trump rallied thousands at an Alabama football field, the super PAC supporting Bush's candidacy flew a plane over Trump's event with a banner hitting the real-estate mogul on his stance on taxes.

Tim Miller, Bush's communications director, attributed the campaign's change of tone to Trump's attempt to grow beyond his status as a policy-light candidate to a serious contender with a real platform.

"As it's become clear that Mr. Trump is trying to run a legitimate campaign rather than a reality-TV stunt, the result is that he has to deal with the scrutiny into one’s record that comes with being a candidate," Miller told Business Insider in an email.

"Jeb is a proven conservative governor who cut taxes, protected life, enacted conservative healthcare reform, while up until a few minutes ago Donald Trump was a liberal Democrat who supported extreme left-wing tax hikes, socialized healthcare, and partial-birth abortion. That is a comparison we welcome," Miller added.

Trump's allies are skeptical of this argument. Roger Stone, the former Trump adviser who has remained a steadfast and increasingly vocal Trump supporter, predicted to Business Insider that Bush's focus on Trump would, incidentally, benefit Trump.

Stone noted the contrast between the pair's dueling town halls in New Hampshire earlier this week. Trump, he said, "looked presidential," with a rambunctious, energetic crowd hanging on the words of an unscripted candidate. Bush's event, he observed, was more "subdued."

(He added that he didn't particularly appreciate Bush's fashion choice at the event. "Where did Jeb get that shirt? Who would ever wear that shirt?")

"I mean, look — for the populist, conservative voters, Trump symbolizes everything wrong with the Republican Party — the country-club Republican Party of Jeb Bush," Stone said. "... The notion that he was going to go to Washington to fight the pampered elite — Jeb, you are the pampered elite."

Bush's campaign against Trump doesn't look to be a short-term effort. A spokeswoman, outlining Bush's week ahead, said he would travel Monday to the border town of McAllen, Texas, where he would participate in a roundtable with local officials to discuss border-security issues.

The spokeswoman noted that Bush "is the only candidate in the presidential field who has put forward a serious, conservative, comprehensive set of solutions to fix our broken immigration system," adding that it "stands in stark contrast to the $500 billion+ plan offered by Donald Trump."

"It works good for Jeb because Jeb's entire strategy is to define himself as someone willing to be independent and hold true to *his* values. Trump is a perfect foil for Jeb. The immigration debate is perfect way to contrast Jeb and Trump's polar views," one veteran Republican strategist told Business Insider.

"I assume the bet is to try to drive Trump to say something so off the reservation it impacts his standing. It may not have immediate success but Bush is well positioned to start putting some blows on Trump."

For its part, the Trump campaign is confident this fight will end up like the others. When Business Insider emailed a Trump spokesman for comment, we noted that Bush is a much stronger candidate than Trump's primary foes so far — Paul, Perry, and Graham.

The Trump team's response: "Is John Ellis different?"