The four lowest-polling Republican U.S. presidential candidates (left to right): former New York Gov. George Pataki, former U.S. Sen. Rick Santorum, Louisiana Gov. Bobby Jindal and U.S. Sen. Lindsey Graham, gather onstage before the second official Republican presidential candidates debate at the Ronald Reagan Presidential Library in Simi Valley, California, Sept. 16, 2015. Reuters

U.S. Sen. Lindsey Graham of South Carolina has a plan to wipe out the Islamic State group. "We are going to kill every one of these bastards we can find, because if not, they are coming here," he told viewers during the second GOP presidential candidate debate Wednesday night.

Graham also said ISIS fighters were already in the U.S. plotting another Sept. 11, 2001-style terrorist attack. Meanwhile, Louisiana Gov. Bobby Jindal called ISIS fighters "barbarians." "We need to do whatever is necessary to hunt down and kill these terrorists," Jindal said during the debate. "These are barbarians … we need to hunt them down and kill them."

Former U.S. Sen. Rick Santorum of Pennsylvania and former New York Gov. George Pataki also participated in the debate, which focused on national security, religious freedom and immigration. The least popular Republican candidates took the stage at 6 p.m. for CNN’s so-called "happy hour" debate Wednesday night at the Ronald Reagan Library debate in Simi Valley, California. A main debate scheduled at 8 p.m. was expected to focus on GOP front-runner Donald Trump.

The other candidates invited to the prime-time event were: Former Florida Gov. Jeb Bush, Wisconsin Gov. Scott Walker, retired neurosurgeon Ben Carson, U.S. Sen. Ted Cruz of Texas, U.S. Sen. Marco Rubio of Florida, former Arkansas Gov. Mike Huckabee, U.S. Sen. Rand Paul of Kentucky; Ohio Gov. John Kasich, New Jersey Gov. Chris Christie and former Hewlett-Packard CEO Carly Fiorina of California.

The first GOP candidate debate in August followed a similar schedule, with Jindal and the other low-polling candidates appearing in an earlier event before a prime-time debate. That "happy hour" debate drew 6 million viewers.

Many Republican voters said Trump is the best candidate to win the 2016 presidential election as the GOP nominee, while they viewed Carson and Bush as the most trustworthy, according to a New York Times/CBS News Poll published Tuesday.