PELLA, Iowa /ANKENY, Iowa (Reuters) - U.S. Republican front-runner Donald Trump expressed confidence on Saturday that he could push back attempts by his rivals to knock him off his top perch, saying he could stand on New York’s Fifth Avenue “and shoot somebody,” and still not lose voters. Nine days from the first nominating contest in Iowa, however, it was Republican rival Marco Rubio who won the endorsement Saturday from the Des Moines Register, the state’s biggest and most influential newspaper. On the Democratic side, the Register picked Hillary Clinton.  The endorsements were big developments for both Rubio and Clinton. Rubio, a Florida senator, has been running third behind Trump and Texas Senator Ted Cruz in Iowa, while Clinton has struggled to fend off a challenge to the Democratic nomination from Bernie Sanders. Trump and Cruz, Trump’s chief obstacle to a victory in Iowa, held competing rallies across the state while in New Hampshire, other candidates battled for votes in that state’s Feb. 9 first-in-the-nation primary for the Nov. 8 election. Trump, the New York billionaire and former reality TV star who has been virtually impervious to attacks from his opponents, pushed the limits of his political rhetoric again in Sioux Center, Iowa. “I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose any voters,” he said.  Trump has been a difficult target for criticism from his rivals because not all of his supporters are conservatives and many are most interested in his projection of strength, not where he stands on a particular issue. The latest Reuters-Ipsos tracking poll had Trump pulling in 40.6 percent support of Republican voters nationally. A CNN/ORC poll has Trump up in Iowa with 37 percent to 26 percent for Cruz, who has led in some other Iowa polls. Trump did not repeat the “shoot somebody” line at a later rally in Pella, while stressing to the crowd there that he would tone down his rhetoric as president. Cruz responded to Trump at an event in Ankeny, where he picked up the endorsement of conservative firebrand Glenn Beck, a counterweight of sorts to Trump’s endorsement by 2008 vice presidential nominee Sarah Palin. “Listen, I will let Donald speak for himself. I can say I have no intention of shooting anybody in this campaign,” Cruz said. Beck was more direct. “There is one thing to have a healthy ego, there is another to give a man who believes those kind of things, who has a habit of anyone who stands in his way of destruction,” Beck said. “To give that man the full power and scope of the office of the presidency is something we will grow to regret.” Iowa Republican Senator Chuck Grassley was an introductory speaker at Trump’s Pella event. Grassley did not endorse Trump but repeated Trump’s signature phrase, saying Republicans have a chance to “make America great again.” During his speech, Trump called Fox News anchor Megyn Kelly biased and said she should not be a moderator at a Fox-hosted Republican debate in Des Moines on Thursday. Kelly’s questioning at an Aug. 6 debate in Cleveland had prompted Trump to unleash a series of insults at her. There was no indication that Fox planned to remove her as a moderator. “Megyn Kelly has no conflict of interest. Donald Trump is just trying to build up the audience for Thursday’s debate, for which we thank him,” said a Fox News spokesperson.  The potential for more chaos in what has been a turbulent race on both the Republican and Democratic sides emerged on Saturday with the news that former New York Mayor Michael Bloomberg might launch an independent run for president. A source said part of Bloomberg’s concern was the problem that Clinton is having in defeating Sanders. “I hope he runs,” Trump told reporters in Pella.  At a First In The Nation forum for candidates in Nashua, New Hampshire, former Florida Governor Jeb Bush was notably withering in his criticism of Trump. He reminded voters of Trump’s dismissal of Senator John McCain as not a hero because he got captured during the Vietnam War. McCain spent 5-1/2 years as a prisoner of war. He was a two-time winner of the New Hampshire primary and the 2012 Republican presidential nominee. “It is not strong to insult women. It is not a sign of strength when you insult Hispanics. It is not a sign of strength when you say that a POW was a loser because they got caught. John McCain is a hero,” Bush said.  (Additional reporting by Dustin Volz in Washington and Alana Wise in New Hampshire; Editing by Sandra Maler and Mary Milliken) SAP is the sponsor of this coverage which is independently produced by the staff of Reuters News Agency.