CHARLESTON, S.C. (Reuters) - Before Donald Trump’s put-downs and the stumbles in presidential debates, and before the profound frustration of voters became so unmistakably apparent, Jeb Bush appeared to be best positioned to win back the White House for Republicans in 2016. He amassed a $150 million war chest in 2015, surrounded himself with some of the best minds in the party, had a famous last name and attracted the support of the party establishment. And yet, after a dismal finish in South Carolina, Bush  dropped out of the race on Saturday with an emotional speech. “I firmly believe the American people must entrust this office to someone who understands that whoever holds it is the servant, not the master, someone who will commit to that service with honor and decency,” he said.     How Bush found himself out of the race after just the third nominating contest of 2016 is a cautionary tale of political miscalculation and strategic errors, according to interviews with a dozen Republican operatives, many with close ties to the Bush campaign and others who worked for the last two Republican presidential nominees, conducted during the last days of his campaign. From the start, they said, Bush appeared to misjudge the mood of the Republican base. In December 2014, for instance, Bush gathered his senior aides and a small group of national political operatives for a meeting in Miami to talk about his coming candidacy. A survey to gauge the national mood of the party was dismissed by Bush and his aides as unnecessary. Such polling, a participant said, would have made clear to Bush the rebellious sentiment of the conservative base of the party.     “They missed the boat,” the participant said. The polling may have also helped the campaign spot the threat of Trump, a billionaire and political outsider who tapped into that anti-establishment anger and stormed to the top of the polls. The Bush campaign disputed the charge as inaccurate. “He has shared the frustration of voters from the outset and he has not strayed from that,” said spokeswoman Kristy Campbell. “He has presented a hopeful, optimistic message that is based on the belief that he has the leadership skills to get the job done.” Viewing Trump as a summer fad who would fade given his outrageous comments, the campaign was slow to respond to the billionaire when he first blasted Bush last August as “low-energy.” Bush maintained his above-the-fray strategy and focused on his policy proposals instead of mixing it up with Trump. Trump relentlessly and bluntly attacked Bush in speeches and on Twitter, portraying him as tired, weak and out of touch with the party. Trump has mentioned Bush on Twitter hundreds of times, far more than any other Republican candidate.      “There was a decision made that he was not a serious person and would fade away,” said one Republican strategist close to the Bush camp, who asked to remain anonymous. “You don’t want to dignify somebody who wasn’t a serious candidate. It wasn’t just Jeb. Nobody thought he was.”     When the “low energy” attacks on Bush started to take hold and his poll numbers began dropping, he went out of his way to insist he had plenty of energy to be president, talking of working 16-hour days, putting in feistier appearances on the stump and hitting back at Trump hard.     Behind the scenes, though, donors said they fretted that too much time had gone by before he took the problem seriously.     Some confidants of the former Florida governor seethed for weeks at Trump’s taunts and urged the campaign to shift to a more aggressive posture.     “They made a horrendous miscalculation in not understanding the intent of the low-energy attack, which was designed to emasculate Bush, to make him look weak,” said Steve Schmidt, who was campaign manager to 2008 Republican nominee John McCain.     “Defining him as weak denied him the ability to make the argument that by resume, competence and experience he was the most fit to command,” said Schmidt.     When Bush launched his campaign in June, he was the clear frontrunner among Republicans, polling at nearly 18 percent in a crowded field. Roughly six weeks later Trump had taken a commanding lead with 26 percent and Bush had dropped to around 12 percent, according to Reuters/Ipsos polling. Today Trump stands at 38 percent.  But Trump was hardly Bush’s only obstacle. Perhaps his biggest misstep was one of his own making, when he spent days trying to explain whether he would have launched the Iraq war begun by his brother, former President George W. Bush.     Even George W. Bush has admitted mistakes were made in the 2003 conflict. But for a week last May, Jeb Bush was flummoxed by the question. The trouble began when he told Fox News that “I would have” launched the invasion even knowing what is now known - that Iraq did not possess weapons of mass destruction. While Bush said he did not understand the question, his response began days of controversy that fed a narrative pushed by Democrats that he was little different from his brother when it came to Iraq.     Some of Bush’s foreign policy advisers were surprised that Bush did not have an answer prepared for the inevitable question.     There was a disagreement among his advisers on how best to respond, “a genuine amount of confusion and upsetness about the whole thing,” said one outside adviser to the campaign, who asked to be anonymous.     Not wanting to be disloyal to his brother, Bush spent days trying to articulate a position until he finally disavowed the statement and declared he would not have ordered the war.     “The fact that we went through this anguished process of several days and several kinds of answers was quite astonishing,” the adviser said.     Finally, George W. Bush told his brother he did not need to defend decisions made back in 2003.  Jeb Bush had an on-again, off-again strategy to embrace the family name, but finally took full advantage of his family in New Hampshire and especially in South Carolina, where the Bush family name remains a valuable brand. For the first time in the campaign former President George W. Bush hit the campaign trail in South Carolina for his brother, addressing thousands of Republicans at a rally.  “I know campaigns are stressful and taxing,” he said. “But they should be. Because the job of the president is much harder than the campaign.” The challenges of the campaign were driven home two days later when Jeb Bush learned that South Carolina’s governor, Nikki Haley, would endorse Rubio.  It was a double blow for Bush: not only did he fail to secure the backing of a popular governor, it went instead to Rubio, his one-time political protégé who has been the target of stinging attacks from Bush. His allied Super PAC, Right to Rise USA, spent at least $12.5 million on advertising opposing Rubio, according to Federal Election Commission data. Bush learned about the endorsement at a town hall meeting in Summerville, South Carolina, where some members of the audience gave him advice on how to run his campaign. He summed up the news briskly. “Disappointed,” he said. By Saturday night, less than two hours after the polls had closed, Bush had called it quits.   (Additional reporting by Grant Smith, Melissa Fares and Chris Kahn; Editing by Paul Thomasch and Ross Colvin) This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.