Democratic U.S. presidential candidates Sen. Bernie Sanders, former Secretary of State Hillary Clinton and former Maryland Gov. Martin O'Malley pose onstage ahead of the second official 2016 Democratic primary debate in Des Moines Saturday. Reuters

Former Maryland Gov. Martin O'Malley directed his insults Saturday at the second Democratic debate not at a rival onstage but one in another party: Donald Trump. While speaking about immigration policy and advocating for a path to citizenship, O'Malley labeled the Republican front-runner and real estate tycoon a "carnival barker."

"You'll never hear this from that immigrant-bashing carnival barker Donald Trump," O'Malley said, his statement peppered with applause, "the truth of the matter is net immigration from Mexico was zero, go ahead, fact-check me." O'Malley was referring to the number of immigrants arriving each year.

The name-calling was not new: During the first Democratic primary debate last month, O'Malley used the same line to take down Trump, who faced criticism in July for referring to Mexican immigrants as drug traffickers and rapists. The billionaire has also proposed building a wall along the border with Mexico to stop illegal immigration.

Trump, true to form, hit back.

O'Malley trailed both former Secretary of State Hillary Clinton and Vermont Sen. Bernie Sanders in the polls Saturday. He had about 3 percent of the vote compared with Clinton's 56 percent and Sanders' 32, according to the survey aggregator HuffPost Pollster.

Sanders strategist Tad Devine told CBS News that the candidate was more prepared this time around. He also revealed that Sanders planned to highlight where his and Clinton's viewpoints diverge. "He understands now that O'Malley -- but also Hillary too -- they have no reluctance coming hard at him. He's now very aware of that. That's going to factor in," Devine said.

O'Malley spokeswoman Haley Morris told CBS News he was "embracing that fight," while Clinton's strategy was likely the same as always -- stay on top and appear likable. “Hillary Clinton looks forward to the debate as an opportunity to discuss her vision for the best solutions to deal with the issues that keep American families up at night," spokeswoman Christina Reynolds told USA Today.

The debate took on a national security focus because it was held about 24 hours after terrorist attacks killed 129 and injured more than 350 people in Paris Friday. The Islamic State group, also known as ISIS, has claimed responsibility for the massacre.