As GOP Nomination Fight Gets More Feisty, So Does Marco Rubio

— -- Following a disappointing fifth place finish in New Hampshire, Marco Rubio is going on the offensive.

Hoping for a better performance in Saturday's South Carolina primary, Rubio has spent the last few days repeatedly blasting Donald Trump, Jeb Bush and Ted Cruz at various campaign stops in the Palmetto State.

"Donald Trump has zero foreign policy experience,” Rubio said in Okatie, South Carolina. “Negotiating a hotel deal in another country is not foreign policy experience.”

He also slammed Trump for recently using a vulgar word to refer to Cruz.

“You turn on your TV and you have the leading presidential candidate saying profanity from the stage. Profanity, from the stage. All these things undermine the things we teach our children,” Rubio said.

Turning to Bush, he accused his former mentor of having “no foreign policy experience, period.”

He assailed Cruz for voting for a federal budget that “bragged about cutting defense spending.”

When reporters later asked Rubio to respond to new attack ads aimed at him from the Cruz campaign, Rubio said Cruz was willing to “say or do anything to get elected. That’s why he ordered his campaign or his campaign ordered people to tell people that Ben Carson was dropping out in Iowa in order to hopefully steal away some votes.”

Rubio’s bare-knuckle approach represents a shift for the Florida senator. Up until recently, he has largely avoided going after other Republicans. But he now says the reason he had a bad debate last week was because he had made the decision not to hit back when New Jersey Gov. Chris Christie attacked his record (Rubio instead repeated the same Obama line four times, which had many mocking his performance as robotic).

“In hindsight, maybe that was a mistake,” he told ABC’s Jon Karl.

"I shouldn’t have done it that way because what it did was it moved me to a message that pivoted away from the question and gave this perception that I tried to evade it,” he said. “The truth is, I just didn’t want to get into a Republican-on-Republican fight, but in hindsight, that probably wasn’t the best way to approach it.”

Rubio will get the chance to make things right for his campaign at the next debate, which takes place tomorrow in Greenville, South Carolina. He maintains that he won’t attack other Republicans “gratuitously” but that he does think it’s time they start discussing their policy differences.

"I’m not going to go in with the goal of attacking other Republicans," he told reporters on Thursday. "But if there’s a policy difference, voters deserve to know that, because they’re trying to make a choice."

He added: "I think now the race is even narrower, the differences are gonna be a little sharper.”