Cruz lambasted Trump for previously describing himself as "very pro-choice."
A fact-checking site could not find evidence of a quote by Stalin about undermining America.
Bush said Trump's "past is littered with failure after failure."
Trump called Cruz a "nasty guy" and mocked his lack of Senate endorsments.
"He wanted more make-up," the aide said of Trump.
He said that under Trump, "your Second Amendment will go away."
Rubio: "I don't know how he knows what I said on Univision because he doesn't speak Spanish."
The two senators battled over an immigration bill.
He gets 42 percent of the debate conversation, followed by Jeb Bush, with 23 percent.
The two are competing for a lane out of South Carolina.