Mitt Romney hopes for another big caucus win in Colorado on Tuesday, as his well-oiled campaign machine works to cement his position as the Republican frontrunner in the US presidential race. Polls suggest he should triumph easily in the western US state -- where he scored 60 percent in 2008 -- although there is a tighter race in Minnesota, where last-placed Rick Santorum is hoping to snatch a win. Romney\'s main rival, former House speaker Newt Gingrich, is battling to stay in the race after two heavy losses in a row to the former Massachusetts governor, who won in Florida last week and Nevada over the weekend. Romney is on a roll in his bid to become the Republican Party\'s nominee to take on President Barack Obama in the general election in November, having taken three of the five state polls so far. His financial and campaigning muscle were on display on the eve of Tuesday\'s caucuses, when he held a rally in a packed school sports hall. \"Mitt, Mitt, Mitt!\" chanted several thousand supporters packed into the Arapahoe High School hall, with others forced to listen to loudspeakers in a neighboring hall as Romney gave a cheer-buoyed version of his stump address. By contrast, Gingrich spoke to a few hundred supporters in a half-full hotel ballroom in Golden, west of Denver, drawing cheers with an attack on a Romney gaffe from last week about poor Americans. \"He said last week that he didn\'t really worry very much about the very poor because after all they had a safety net,\" Gingrich said, calling Romney\'s verbal stumble \"backwards\" in terms of traditional conservative thinking.  The candidates are battling for the biggest share of the 1,144 delegates at the Republican Party\'s convention in Tampa, Florida, in August. With Nevada, Romney has 99, against 33 for Gingrich, 15 for Paul and 11 for Santorum. Tuesday\'s non-binding caucuses in Colorado and Minnesota will be the last contests before Maine\'s week-long straw poll begins on February 11 and Michigan Republicans vote in a February 28 primary. The caucuses begin at 7 pm (0100 GMT in Minnesota and 0200 GMT in Colorado) and results may not immediately be available. Full results for Nevada published on Monday gave Romney 50 percent of the vote, more than twice that of his nearest rival Gingrich, with 21 percent. Paul came in third with 19 followed by Santorum with 10. In Colorado Romney\'s win might not be crushing, but should be comfortable: a Public Policy Polling (PPP) survey on Sunday put Romney at 40 percent, Santorum at 26, Gingrich at 18 and Paul at 12. While the Colorado result is barely in doubt, Romney could see his momentum slowed by a narrow victory -- or even a loss -- in the heartland state of Minnesota. He was in a statistical tie with conservative darling Santorum, who is hoping to carry Minnesota\'s caucuses on Tuesday with the help of the state\'s large base of evangelical Christian and ultraconservative Tea Party voters.  Santorum, who has seen his campaign flag after scoring an early victory in Iowa\'s first-in-the-nation caucus, is hoping for another upset in Minnesota\'s non-binding caucus, where turnout is expected to be low. Romney -- who carried Minnesota in 2008 with 41 percent of the vote -- has not returned to the midwestern state since last week, relying instead on former Minnesota Governor Tim Pawlenty to press his cause. \"The only candidate in the race who hasn\'t spent the bulk of his adult life either in Washington, DC or in a parasitic relationship with Washington, DC is Mitt Romney,\" Pawlenty told a group of around 40 supporters gathered at Romney\'s suburban Minnesota headquarters. As the Republicans slugged it out, Obama saw his approval rating soar Monday to the 50-percent mark, with a solid lead over Romney, seen as his likely Republican opponent in November. Obama leads Romney 52 to 43 percent among all Americans, and a narrower 51 to 45 percent among registered voters, according to a poll by the Washington Post and ABC television.