The GOP presidential debate Thursday night kicked off with early fireworks as Donald Trump and Marco Rubio quickly began bickering with each other. 

The two were at each other's throats within minutes, as Trump and Rubio shouted over each other over whether Trump could beat Hillary Clinton in a general election.

Trump at one point referred to his penis size while countering against attacks from Rubio. 

"He hinted my hands are small, something else must be small. I guarantee you there's no problem," Trump said.

Rubio was asked why he changed his campaign's tone to begin making personal attacks against Trump in the last week.

"If there's anyone who's ever deserved to be attacked that way, it's Donald Trump," Rubio said.

"This little guy has lied so much about my record," Trump countered.

"Have you ever heard of Trump Steaks?" Rubio asked, grinning. "He's not going to make America great."

But Fox News anchor Chris Wallace threw some of Trump's arguments back in his face. He argued that slashing the Department of Education's budget of $78 billion and the Environmental Protection Agency's $8 billion wouldn't make up the country's $544 billion deficit, while Medicare only spends $78 billion on medicine, Trump said he could save while $300 billion by re-negotiating Medicare medicine deals.

"I'm not talking just about drugs, I'm talking about other things. We'll save $300 billion a year if we properly negotiate," Trump responded.

Ted Cruz argued that his campaign offers more to voters. 

"I understand the folks who support Donald. You're angry, you're angry with Washington," Cruz said, adding that "Donald has been part of the corruption" for 40 years. 

"They're interested in solutions, not slogans ... The question is, do you understand the principles that made America great in the first place?" Cruz asked. 

Meanwhile, fourth-place nominee John Kasich barely got a word in edgewise after referring to himself as the "only adult on the stage."

Trump also used the debate to counter two of the biggest stories about him in recent days. He disavowed the Ku Klux Klan and David Duke from the stage, and lashed back against former GOP candidate Mitt Romney, who criticized Trump Thursday: "He was a failed candidate, he should have beat President Obama very easy. He failed miserably and it was an embarrassment to everybody." 

The debate also marked the apparently friendly reunion of Fox News anchor Megyn Kelly and Donald Trump.

"Mr. Trump, hi, hello," Kelly said in her first questions to him.

"Nice to be with you Megyn, looking well," Trump responded.

"As are you," she said.