WASHINGTON - Mitt Romney clinched the Republican presidential nomination on Tuesday with a resounding victory in Texas and now faces a five-month sprint to convince voters to trust him over Democratic President Barack Obama in the November 6 election.Although the race has been essentially over for weeks, Romney finally cleared the necessary benchmark of 1,144 delegates for becoming the Republicans' presidential candidate after a long, bitter primary battle with a host of conservative rivals.This argument will move soon to the energy industry, which Romney thinks Obama has bungled by not ramping up domestic production of oil and natural gas.Romney in weeks ahead will turn to Obama's 2010 healthcare overhaul. The US Supreme Court is to decide in late June on the constitutionality of the law's requirement that all Americans purchase health insurance.Romney has vowed to repeal the law if elected, citing it as an example of too much government under Obama. He has faced criticism from Republicans for the healthcare overhaul he developed for Massachusetts that Obama has called a model for revamping the US system.Winning the nomination put to rest any lingering suggestion that Romney could face a conservative challenge at the Republican convention in Florida in late August as Gingrich had threatened to do when the race was still close.Romney must overcome conservative base's warinessRomney is trying to overcome wariness among conservatives, who mistrust his record in Massachusetts where he introduced a healthcare reform that they say was a blueprint for Obama's 2010 US healthcare overhaul program that was approved in Congress despite heavy Republican opposition."I was looking forward to voting for Rick Santorum," said voter Dan Cortez in San Antonio. He said he would now back Romney, for he believes it is important to elect "anybody who can beat Obama."Texan Republicans on Tuesday also choose their candidate for a US Senate race in November. Lieutenant Governor David Dewhurst faces former state Solicitor General Ted Cruz in the primary. It will end up in a July runoff if neither man can reach 50 percent support.He will be formally nominated at the Republicans' convention in Florida in late August.In a statement, Romney said he was humbled to win enough of Texas' 155 delegates to secure the nomination."Our party has come together with the goal of putting the failures of the last three and a half years behind us. I have no illusions about the difficulties of the task before us. But whatever challenges lie ahead, we will settle for nothing less than getting America back on the path to full employment and prosperity," he said.While the milestone should create positive buzz for his campaign, Romney's big day was overshadowed by his appearance with real estate tycoon and reality TV star Donald Trump, who organized a major fund-raiser for Romney in Las Vegas. Romney endured serious threats from Republican opponents from Rick Perry to Rick Santorum to reach a goal that his late father, former Michigan governor George Romney, fell short of achieving -- win his party's stamp of approval as its presidential candidate.He is considered the underdog in his battle with the Democratic incumbent but all indications are that Americans face the possibility of a cliffhanger election in November that will be decided by relatively small percentages of voters in as many as a dozen battleground states.The former Massachusetts governor now faces a lengthy to-do list to gird for his duel with Obama, from picking a vice presidential running mate to raising hundreds of millions of dollars for a national campaign.In the immediate weeks ahead, his goal is to bolster his case that Obama has been ineffective in handling the sluggish US economy and hostile to job creators.