CHARLESTON, S.C. (Reuters) - Long viewed by the Republican establishment and many donors as one of their rising stars, U.S. Senator Marco Rubio has been struggling to make his mark in the race for the party’s 2016 presidential nomination in a field led by mavericks Donald Trump and Ted Cruz. Regarded as well-spoken and telegenic, Rubio began facing criticism a few months ago when he was perceived as campaigning too little in Iowa and New Hampshire, the two states where next month party members will cast their first ballots. Rubio, 44, may still have a breakthrough, but with 20 days to go until Iowa’s Feb. 1 nominating caucuses, the first-term Florida lawmaker faces questions about his campaign strategy in a race for the Nov. 8 election made up of 11 candidates dominated by the billionaire Trump and U.S. Senator Cruz of Texas. His prospects reach a critical moment at a Thursday night debate in South Carolina when he tries to position himself as the consensus alternative to Trump and Cruz and distance himself from such establishment candidates as former Florida governor Jeb Bush and governors Chris Christie of New Jersey and John Kasich of Ohio. Republican experts say he seems to be straddling two political lanes, trying to win over establishment Republicans as well as anti-establishment conservatives who favor Cruz. The debate, on the Fox Business Network, is due to begin at 9 p.m. ET on Thursday (0200 GMT on Friday). The son of Cuban immigrants, Rubio has drawn support by stressing his working-class upbringing in Las Vegas and Miami as the child of a bartender father and a mother who was both a maid and a cashier. But voters and party operatives suggest he has until recently failed to commit to the intimate face-to-face politicking considered mandatory by voters in either Iowa or New Hampshire, which has a Feb. 9 primary. Rubio aides acknowledge the high stakes of the coming weeks. “I feel good but we’ve got to close strong,” said Rubio’s New Hampshire state director, Jim Merrill. A Reuters/Ipsos opinion poll showed Rubio on Wednesday at 6.7 percent, compared to 15.2 percent little more than a month ago, falling from second place to fifth among Republicans after Trump, Cruz, Bush and retired neurosurgeon Ben Carson. Some voters have cited a tendency by Rubio in past months to stick to well-honed talking points on the stump instead of mixing it up at town-hall meetings where people can ask questions. This has raised concerns he might lack necessary experience. “There’s a lot of people who were very interested in him and continue to be, but in Iowa it just seems like he hasn’t fully committed himself to aggressively campaign in the state and that’s causing some pause,” Iowa Republican strategist Doug Gross said. Dennis Thorell of New Durham, New Hampshire, said Rubio needed a longer resume of accomplishments than U.S. senator and former speaker of the Florida House of Representatives. Thorell said Rubio might be better suited for the vice presidency or another top job in a Republican administration. “He hasn’t had any experience running anything,” Thorell said while listening to Rubio in Concord, New Hampshire, last week. “He did very well in the legislature down in Florida and was mentored by Jeb (Bush) but he hasn’t run anything.” Rival candidates for the Republican nomination have assailed Rubio also for his absences from the U.S. Senate during hearings and floor votes, raising further questions about how he spends his time. Defending his record of missed votes, Rubio has said the majority of his time is spent in service to his constituents and his Senate committee work, both of which he says continue unabated. Rubio aides told Reuters they plan a multimillion-dollar advertising campaign for New Hampshire and expect to outspend all rivals on the TV airwaves in the state. The aides said Rubio has plans to be a major presence in both Iowa and New Hampshire in the final stretch, with a schedule that will include plenty of give-and-take with voters in informal settings such as town halls. They said Rubio’s schedule is as heavy as any of his rivals, but that much of it has been at behind-the-scenes fund-raising events that have been essential to get him the resources needed for the final push. Republican strategists say Rubio’s path to the nomination would have to start with at least a third-place finish in Iowa presumably behind poll leaders Cruz and Trump, which would deliver him some momentum going into New Hampshire. If Rubio goes on to place high in Iowa and do well in New Hampshire, he and his aides will see it as the realization of a strategy of going for broke at the right moment in the states that render the first judgments in the presidential race. “From the get-go we said we didn’t want to peak in September, we wanted to peak in February. It’s starting to grow. It feels like it’s starting to happen for us,” Rubio spokesman Alex Conant said. Republican analysts say Rubio appears to be as well-placed as any of his rivals in the drive to become the establishment Republican favorite. He is drawing increasingly larger crowds in New Hampshire and has a solid organization in the state. “I think he suffered from the perception that he hasn’t been here as much and worked it as hard as some of his competitors,” Republican strategist Steve Duprey said. “If he puts the pedal to the metal, he could do very well.” For more on the 2016 presidential race, see the Reuters blog, “Tales from the Trail” (here). 
For Republican Rubio, a moment of truth in race to lead U.S.