The Republican primary just reached new levels of nastiness ahead of the big debate

The Republican presidential race has reached new levels of infighting ahead of the Tuesday night debate hosted by the Fox Business Network.

Real-estate mogul Donald Trump, naturally, led the way with the most personal insults.

At a Monday night rally in Springfield, Illinois, Trump attacked both Sen. Marco Rubio (R-Florida) and retired neurosurgeon Ben Carson, two of his leading opponents in the polls. 

Trump blasted both for being "very, very weak" on illegal immigration. He also hit Rubio for his poor Senate-attendance record and for drinking too much water during his 2013 response to the president's State of the Union address.

"I don't care if he's a nice guy. Remember when he was doing the message to the president? Remember the thing with the water? This guy gets the greatest press. So he's sweating and sweating, pouring sweat," Trump recalled. "I notice. I say,, 'Man is he sweating!'" 

And some of the top-tier candidates who have thus far largely declined to blast their rivals are increasingly taking their gloves off, as well.

Rubio hit back quickly after The New York Times reported Monday night that former Florida Gov. Jeb Bush's (R) big-money super PAC was testing anti-Rubio messaging and boasting that it could spend as much as $20 million to take Rubio down a notch.

The senator's operation responded by releasing a video reminding viewers that Bush once vowed to run a "joyful" campaign and that Bush once used to shower praise on Rubio as a potential vice-presidential pick in 2012.

Bush and Rubio, a former Florida house speaker, were allies in the Tallahassee statehouse. Some Bush supporters have even reportedly taken to calling Rubio "Judas" because of his lack of deference to Bush in the race. 

Here's the Rubio video:

Rubio's campaign manager, Terry Sullivan, fired off a fundraising solicitation to supporters based on The Times' report. 

"This is a Joyful Campaign???" Sullivan asked in the email's subject line. "Jeb's plan is to spend $20 million dollars to 'damage Mr. Rubio's reputation. Not just talk about how they differ on the issues but to actively try to destroy Marco's reputation. And they're bragging about that? How is that the kind of "joyful" campaign that Jeb claimed he wanted to run?!?!"

Sullivan continued: "It gets better. Jeb's Super PAC has even produced a video claiming that Marco is too Pro-Life!!! Honestly, it's something we'd expect from Hillary." 

Rubio also took a jab at Trump during a Monday night Fox News interview. Asked about Trump attacking his personal finances as a "disaster," Rubio pointed to Trump's past corporate bankruptcies. 

"I'm not sure someone who's like Donald Trump, that's taken four companies into bankruptcy, should be lecturing anybody about finances," Rubio said.

Even the famously mild-mannered Carson has started to return fire at Trump, who repeatedly taunted him over the weekend for supposedly having an incurable "pathological disease."

Trump jumped on last week's media reports questioning statements made in Carson's memoir, "Gifted Hands." Among other things, a CNN investigation could not find anyone who could corroborate Carson's stories of violent childhood incidents, which Carson said ultimately led to him disavowing his temper and becoming the more mild-mannered person he is today. But Trump accused Carson of having a pattern of lying.

"WOW, one of many lies by Ben Carson! Big story," Trump tweeted Friday, after Politico reported that Carson had exaggerated a supposed scholarship offer from West Point.

Carson aggressively pushed back against the reports and questioned Trump's character for not waiting to find out the facts.

"What does it say about people who immediately jump on the bandwagon if they hear something bad rather than waiting and finding out what the truth is?" Carson asked Sunday on ABC's "This Week."

He added: "Let me put it this way: I would not be anxious to have a commander-in-chief who acted that way."

Trump gave a Tuesday interview on on ABC's "Good Morning America," during which he was asked about Carson's response. Trump promptly unloaded on Carson for the violent incidents described in his memoir.

"I'll just simply say and I mean it to the bottom, the case was made that he went after his mother with a hammer and he wanted to hit his mother over the head with a hammer, that he stabbed somebody, that he … smashed somebody's face with a lock - with a padlock - and other things. And he's trying to justify this because he wants to be telling the truth," Trump said.

"Then he said he's pathological, he's got a pathological temper or something, and he wrote that himself in the book," Trump added. "I'm trying to figure out what's going on over here. You hit somebody in the face with a lock. You go after your mother with a hammer. You do all these things, and you're trying to justify that, yes, you did them and that's supposed to make you credible. And it's a very strange situation. " 

Other potential feuds are also simmering ahead of the Tuesday night debate: 

• Bush and Trump - who have repeatedly exchanged shots on the campaign trail and during debates - have made no secret their disdain for one another.
• Ohio Gov. John Kasich (R) and Trump tangled during the last debate, as have Sen. Rand Paul (R-Kentucky) and Trump for weeks.
• And Sen. Ted Cruz (R-Texas) has taken some subtle shots at Rubio as they have both risen in the polls. 

All this personal and political animosity could leave the Fox Business debate a potential powder keg ready to explode, though the moderators have promised to have a more issues-focused affair than the CNBC debate last month.

"CNBC never asked the real questions, never covered the real issues," a narrator said in a Fox Business promotional video ahead of the debate.