U.S. Republican presidential candidate and businessman Donald Trump speaks at a campaign rally at the South Point Resort and Casino in Las Vegas, Nevada January 21, 2016.

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’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.

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