Donald Trump is facing a new type of threat from 2 surging candidates

Almost every Republican presidential candidate until recently has floundered when attacking real-estate tycoon Donald Trump.

But since last week, two surging White House hopefuls are taking a markedly different approach: former Hewlett-Packard CEO Carly Fiorina and Sen. Marco Rubio (R-Florida).

Rather than regularly berating the GOP front-runner or pretending he doesn't exist, Fiorina and Rubio have been biding their time and, when the opportunity strikes, delivering a limited- and arguably emasculating - attack.

When the inevitable flood of Trump counter-punches followed, they have quietly held their ground without escalating the situation.

And so far, at least, it appears to be working - though Trump argues they're doomed to fail like everyone else who attacks him.

"Every single person I've talked about has gone down and I never do the talking first. I'm always counter-punching," Trump told Business Insider on Thursday. "Rubio is going to be down too."

However, there may be a small amount of evidence that the Rubio-Fiorina strategy is effective. Fiorina probably demonstrated it the best during the September 16 CNN presidential debate, after which she soared in polls and is now competing for the No. 2 position behind Trump.

It was her first time on the main stage, as she was relegated to the "kiddie table," lower-tier affair of the Fox News debates the previous month. And Fiorina was at the center of the latest Trump controversy at the time: In a Rolling Stone profile published a week before, Trump was quoted seemingly mocking her appearance.

"Look at that face," he said, according to Rolling Stone. "Would anyone vote for that?"

Trump later insisted he was talking about Fiorina's "persona," not her looks, but the narrative had already been cemented. And given the opportunity to confront Trump over the Rolling Stone comments at the CNN debate, Fiorina steeled herself and suggested he was being sexist.

"I think women all over this country heard very clearly what Mr. Trump said," Fiorina said to loud applause.

The shiv had apparently made its way through the Trump armor. Instead of the typical counterpunch, he responded with a half-hearted compliment: "I think she's got a beautiful face, and I think she's a beautiful woman."

Rich Lowry, the editor of the conservative magazine National Review, infuriated Trump while describing that debate moment during a Fox News interview last Wednesday. Speaking with host Megyn Kelly, Lowry crudely declared that Fiorina had castrated the front-runner.

"Trump obviously attacks everyone. But she's become a much bigger target. And I think part of what's going on here is that last debate. Let's be honest: Carly cut his balls off with the precision of a surgeon, and he knows it," Lowry said.

Kelly was shocked.

"What did you just say?!" Kelly said. "You can't say that."

But Trump wasn't satisfied with her admonishment. He quickly exploded on Twitter. He described Lowry as "incompetent" and "clueless," demanded an apology from Fox News, and suggested that the Federal Communications Commission should fine Lowry.

During a interview on CNN the next day, Trump offered still more criticism. He said Lowry "used words that were so unbelievable," "made a total fool of himself," and had a "nervous breakdown on television."

Here's Kelly reacting to Lowry's comment:

It seems Rubio took a page out of Fiorina's book.

After the big CNN debate, where a strong performance also seemed to jump-start Rubio's poll numbers, Rubio suddenly kicked his criticism up a notch. During a Fox News interview last Tuesday, the senator started directly bashing Trump on a single topic: national security.

"I think that the most import thing the president will ever do is provide for the national security of our country. And I think up until now, he hasn't answered serious questions about national security. And until he does, there should be concerns - not just about him - but about any candidate that's not able to speak in detail, with clarity, and with seriousness about the national-security threats that we face," Rubio said.

He also said that Fiorina had more foreign-policy knowledge than Trump.

"As of now I haven't heard him talk seriously about national security. Hopefully that will change," he added. "Look, Carly Fiorina's not in government - she knows more about it than he does."

Politico's Marc Caputo recently reported on Rubio's tactical shift against Trump and suggested that Fiorina's success may have played a role.

"She proved last week that Trump's not invincible, landing a solid blow during the CNN debate," Caputo wrote. "She showed others that the counter-counter-punch can be pretty sweet, as shown by her surge in the polls."

Trump responded to both Fiorina and Rubio with a flurry of attacks.

In campaign-trail speeches and interviews, Trump now constantly trashes Fiorina's "horrible" tenure at Hewlett-Packard, which he said caused her to lose her 2010 US Senate race in California. He's even repeatedly called her voice grating.

"I think that she's got a good line of pitter-patter. But when you listen for more than five minutes, you develop a tremendous headache," Trump said last Monday on "Fox & Friends."

Trump's response to Rubio's foreign-policy broadside was similar. In a throwing-spaghetti-against-the-wall attack, Trump has repeatedly dismissed the 44-year-old Rubio as a "kid," attacked him for his poor Senate-attendance record, dismissed his poll numbers, criticized him for being disloyal to a political mentor, and even mocked him for sweating too much at the debate, among other things.

"Obviously, I'm not a fan of Marco Rubio from the standpoint that I think he talks quickly, but he doesn't talk intelligently," Trump explained to Business Insider.

Rubio fired back in an interview Thursday, seemingly trying to get under Trump's skin with a remark about his debate performance and his "touchy and insecure" reaction to criticism.

"He had a really bad debate performance last week," Rubio told Kentucky Sports Radio, according to BuzzFeed. "He really never talks about issues and can't have more than a 10-second soundbite on any key issue. And I think he's kind of been exposed a little bit over the last seven days."

Trump has continued jabbing at Rubio - during a speech on Friday, Trump called Rubio a "clown."

Rubio and Fiorina's more measured approach to criticizing Trump is noticeably different from how their rivals have treated the front-runner.

Former Texas Gov. Rick Perry (R) furiously attacked Trump, dedicating an entire speech to calling him a "cancer" on the conservative cause, firing off harsh press releases about his policy positions and rhetoric, and even fighting with him on social media.

Wisconsin Gov. Scott Walker (R), meanwhile, went out of his way to ignore Trump's taunts, which started after he declined to publicly chide a supporter who called Trump a "DumbDumb." Trump responded to the insult by trashing Walker's gubernatorial record as a complete disaster.

Both Perry and Walker recently suspended their campaigns and dropped out of the race.

Former Florida Gov. Jeb Bush (R), Sen. Lindsey Graham (R-South Carolina), Louisiana Gov. Bobby Jindal (R), Sen. Rand Paul (R-Kentucky), and former New York Gov. George Pataki (R) all appear to be playing from the Perry playbook of frequent Trump confrontations. As the mogul likes to point out, they have not exactly seen their campaigns take off as a result.

"If you look at the list, every single - Jindal is down, gone. They're all gone. Pataki is gone. Lindsey Graham is gone. Walker is gone. And they always start the talking, and I finish it," Trump told Business Insider.

The Walker approach has had a bit more success. Retired neurosurgeon Ben Carson, the soft-spoken candidate who often polls at No. 2 in the primary, has done well despite only taking one shot at Trump's lack of public religiosity and humility. After Trump unloaded on Carson in response, he backed off and so did Trump.

Others like Ohio Gov. John Kasich (R) and Sen. Ted Cruz (R-Texas) have also stood out - by praising Trump when asked about him.