Mitt Romney has clinched the Republican presidential nomination with a win in the Texas primary.
An Associated Press delegate count showed Mr Romney had passed the 1,144 delegates needed to win the nomination during the primary. Early returns show Mr Romney posting a big win in Texas.
It's a triumph of endurance for a candidate who came up short four years ago and had to fight hard this year as voters flirted with a carousel of Republican rivals.
Mr Romney reached the nomination milestone with a steady message of concern about the US economy, a campaign organisation that dwarfed those of his Republican opponents, and a fundraising operation second only to that of his Democratic opponent in the general election, president Barack Obama.
Mr Romney said: "I am honoured 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.
"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 outlasted a carousel of Republican rivals who dropped out of the state-by-state primary contest. None of his former rivals actively campaigned in Texas.
Mr Romney must now energise conservatives who still doubt him, while persuading undecided voters that he can do a better job fixing the nation's struggling economy than Mr Obama.
In Mr Obama, he will face a well-funded candidate with a proven campaign team in an election that will be heavily influenced by the economy.
Republicans will not officially nominate Mr Romney until late August at the Republican National Convention in Tampa, Florida.
He won at least 88 delegates in Texas with 64 left to be decided, according to early returns. The 152 delegates in Texas are awarded in proportion to the statewide vote. Mr Romney now has 1,174 delegates.
Mr Romney, 65, is clinching the presidential nomination later in the calendar than any recent Republican candidate - but not quite as late as Mr Obama in 2008. He clinched the Democratic nomination on June 3, 2008, at the end of an epic primary battle with Hillary Clinton. Four years ago, John McCain reached the threshold on March 4, after Mr Romney dropped out of the race about a month earlier.
Several other Republican contenders - including Newt Gingrich, a former speaker of the US House of Representatives, and Rick Santorum, former US Senator from Pennsylvania - earlier dropped out of the race as Mr Romney's well-financed campaign gained momentum.
He has been in general election mode for weeks, raising money and focusing on Mr Obama, largely ignoring the primaries since his competitors dropped out or stopped campaigning.
Libertarian-leaning Texas candidate Ron Paul said on May 14 he would no longer compete in primaries, though his supporters are still working to gain national delegates at state conventions.