Mitt Romney has clinched his Republican party's White House nomination by winning its Texas primary, vowing to get America "back on the path to prosperity" by defeating Barack Obama in November.
But Tuesday's milestone was clouded by a revived controversy over claims by billionaire tycoon Donald Trump, a high-profile Romney supporter, questioning where Obama was born, and therefore his right to be president.
"I am honored that Americans across the country have given their support to my candidacy and I am humbled to have won enough delegates to become the Republican Party's 2012 presidential nominee," Romney said.
"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."
Romney, the only candidate who actively campaigned in Texas, won 69 percent of the vote, against 12 percent for Ron Paul, 8 percent for Rick Santorum and 5 percent for Newt Gingrich, according to CNN, with 99 percent of votes counted.
Texas, the second most populous US state, had 155 delegates up for grabs which—added to former Massachusetts governor's tally of 1,064—takes him well over the nomination threshold of 1,144 delegates.
But while Romney celebrated confirmation that he will take on Obama in November 6 polls, the campaign risked veering off message due to interventions by flamboyant real estate tycoon Trump, who endorsed the candidate in February.
Trump—with whom Romney attended a fundraiser in Las Vegas as the Texas results came in late Tuesday, spent much of the day insisting there were still lingering doubts about whether Obama was really a natural born US citizen.
"Nothing has changed my mind," he told CNBC about the so-called "birther" issue, adding: "There are some major questions here and the press doesn't want to cover it."
That provided an opening for Obama's deputy campaign manager Stephanie Cutter to slam Romney for lacking "moral leadership" over his appearance with Trump.
"If Mitt Romney lacks the backbone to stand up to a charlatan like Donald Trump because he's so concerned about lining his campaign's pockets, what does that say about the kind of president he would be?" she said.
Romney's campaign was forced into awkward damage control hours before the two men appeared together, with spokeswoman Andrea Saul saying Romney "has said repeatedly that he believes President Obama was born in the United States.
"The Democrats can talk about Donald Trump all they want—Mitt Romney is going to talk about jobs and how we can get our economy moving again."
Both Trump and Romney avoided commenting on the birther row at Tuesday evening's fundraiser at the tycoon's Vegas hotel-casino, which was reportedly expected to generate $2 million for the candidate's campaign.
Romney, 65, pivoted toward Obama in his campaign speeches and events more than a month ago, when it became clear his long march toward the nomination at the party convention in August would not be stopped.
But that was only after a bruising primary race, in which rivals like Gingrich and Santorum slowed Romney's rise by rallying more conservative voters and highlighting his flipflops on issues such as abortion.
Polls show a steadily tightening White House race, with Republicans coalescing behind Romney in the weeks since Gingrich and Santorum dropped out.
Texas Governor and former candidate Rick Perry, who had also bitterly sparred with Romney, congratulated him on his victory Tuesday.
Poll aggregates show Obama narrowly ahead. The latest RealClearPolitics average shows the president with a two-point lead, 45.6 to 43.6 percent.
Republican National Committee (RNC) chairman Reince Priebus hailed Romney's Texas win, saying it paves the way for the party's August convention in Tampa, Florida, where Romney will be formally nominated and reveal his running mate.
"Governor Romney will offer America the new direction we so desperately need. We cannot afford four more years of President Obama's big government agenda, deficit spending, and attacks on American free enterprise."
Romney meanwhile expressed confidence that the country would unite behind his candidacy on 6 November, in order to "begin the hard work of fulfilling the American promise and restoring our country to greatness."