Mitt Romney 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 the milestone was clouded by a rehashed controversy over claims by billionaire tycoon Donald Trump, a high-profile Romney supporter, questioning President Obama's birthplace.
"#1144. Thank You. Whatever challenges lie ahead, we will settle for nothing less than getting America back on the path to prosperity," Romney tweeted, referring to the number of delegates required to win his party's nomination.
The former Massachusetts governor, the only candidate who actively campaigned in Texas, won 71 percent of the vote, according to Fox News, CNN and NBC television.
US congressman from Texas Ron Paul won 10 percent in his home state, Catholic conservative Rick Santorum 7 percent and former House Speaker Newt Gingrich 5 percent, according to CNN.
In Texas 155 delegates were at stake -- which added to Romney's tally of 1,064 should take him well over the 1,144 nomination threshold, according to the website RealClearPolitics.
But while Romney celebrate the achievement, the campaign risked veering off message thanks to interventions by flamboyant real estate tycoon Trump, who endorsed the candidate in February.
Trump -- with whom Romney was attending a fundraiser in Las Vegas as the Texas results came in, spent much of Tuesday 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 campaign 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?" said Obama's deputy campaign manager Stephanie Cutter.
Romney's campaign was forced into awkward damage control hours before the two men appear 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."
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.
"Gov. 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.
In nominating a multimillionaire former businessman, the Republican Party is in familiar territory, but in one key respect Romney is making history, as the nation's first-ever Mormon nominee of a major political party.
The Republican base has long been dominated by evangelical Christians, and Romney's faith has occasionally come under scrutiny by some religious leaders.
But Romney is counting on Americans seeing him as the pragmatic problem solver with the business credentials to turn the economy around better than Obama has.
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 it was a brutal primary season. Rivals like Gingrich and Santorum humbled Romney by stealing some victories, rallying voters to their more conservative agenda and highlighting his flipflops on key 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.
Poll aggregates show Obama narrowly ahead. The latest RealClearPolitics average shows the president with a two-point lead, 45.6 to 43.6 percent.