Republican front-runner Donald Trump sharply clashed with challenger Jeb Bush in a national security debate on Tuesday but escaped any severe criticism from his other rivals, who were reluctant to take on the contentious billionaire. Bush, who has lagged in opinion polls, assailed Trump for a lack of depth and seriousness and called him a "chaos candidate" who was adept at one-liners but naive on policy issues. He said Trump's proposed ban on Muslims entering the United States was not a serious plan. The other contenders, including U.S. Senator Marco Rubio of Florida and U.S. Senator Ted Cruz of Texas, avoided criticism of Trump and focused their attacks on President Barack Obama and Democratic front-runner Hillary Clinton, Obama's former secretary of state, accusing them of failing to keep the United States safe. "Donald is great at the one-liners, but he is a chaos candidate and he would be a chaos president," said Bush, 62, the former governor of Florida, who called Trump's proposal to kill the families of Islamic State militants "just crazy." "Donald, you're not going to be able to insult your way to the presidency," Bush said. "Leadership is not about attacking people and disparaging people." Trump, 69, dismissed Bush's criticism. "With Jeb's attitude we will never be great again," Trump said, and defended his Muslim proposal. "We're not talking about isolation; we're talking about security. We're not talking about religion; we're talking about security. Our country is out of control," Trump said. When Trump complained that the CNN moderators were trying to entice candidates into attacking him, Bush said "this is a tough business, running for president." 'A TOUGH GUY' "Oh yeah, and you're a tough guy, Bush," Trump said, noting his falling standing in the polls and his resulting move down to the end of the debate stage. "You're moving further and further," Trump said to Bush. "Pretty soon, you're going to be off the end." Trump, who also backed closing parts of the Internet used by Islamic State militants, has dominated the last few weeks of the campaign with his call for a total ban on Muslims entering the United States, following a married couple's Dec. 2 massacre of 14 people in San Bernardino, California, inspired by Islamic State. Most of Trump's Republican rivals, as well as officials and leaders in the United States and around the globe, have criticized Trump's proposal although many of his supporters have stood by him. Aside from Bush, candidates sidestepped opportunities to take on Trump, although they did not hesitate to tangle with one another. Cruz and Rubio, both 44, battled over Cruz's proposal to "carpet-bomb" areas of the Middle East controlled by Islamic State militants, also known as ISIS. "ISIS is gaining strength because the perception Is that they're winning, and President Obama fuels that perception," Cruz said. "That would change when militants across the globe see that when you join ISIS you are giving up your life." But Rubio noted that Cruz had voted against defense authorization bills and had supported defense cuts that would make such bombing impractical. "You can't carpet bomb ISIS if you don't have planes or bombs to attack them with," he said. IOWA SEVEN WEEKS AWAY With seven weeks to go before the first nominating contest in Iowa, Trump, has held or expanded his lead in national polls in the Republican race for the November 2016 presidential election. Cruz has moved past him in some Iowa surveys. Cruz has been reluctant to criticize Trump, and while Trump called Cruz "a maniac" at the weekend, he passed up the chance to do it again. "He's just fine, don't worry about it," Trump said, patting Cruz on the back when the subject arose. The Republican debate on Tuesday was the first since the attack in California and another in Paris, and focused heavily on national security. The latest Reuters/Ipsos opinion poll, completed after Trump's Dec. 7 call for a ban on Muslims, showed him leading the field with support of 33 percent of Republican voters. Cruz was second at 15 percent, followed by retired neurosurgeon Ben Carson at 12 percent, Rubio at 10 percent and Bush at 9 percent. Also appearing in the main debate were New Jersey Governor Chris Christie, Ohio Governor John Kasich, U.S. Senator Rand Paul of Kentucky and former corporate executive Carly Fiorina. At the so-called "undercard" debate of low-polling candidates, held before the main event, former New York Governor George Pataki blasted Trump's Muslim proposal as "un-American, unconstitutional, and it is wrong." Lindsey Graham, a U.S. senator from South Carolina with hawkish views on national security, was especially withering. "Donald Trump has done the one single thing you can't do: Declare war on Islam itself," Graham said, calling the proposal a coup for Islamic State militants trying to recruit new members. Former U.S. Senator Rick Santorum of Pennsylvania and former Arkansas Governor Mike Huckabee said, however, that Trump had brought up an important issue, stressing the need to protect American citizens from Islamic State militants. "He has touched a nerve because people are angry and afraid," Huckabee said. Graphic on key points made by candidates during debate: here
