Mitt Romney clinched the Republican presidential nomination Tuesday with a win in the Texas primary, 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 party rivals.

According to The Associated Press count, Romney surpassed the 1,144 delegates needed to win the nomination by winning at least 88 delegates in the Texas primary.

The former Massachusetts governor has reached the nomination milestone with a steady message of concern about the U.S. economy, a campaign organization that dwarfed those of his Republican rivals and a fundraising operation second only to that of his Democratic opponent in the general election, President Barack Obama.

"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," Romney said in a statement.

"Our party has come together with the goal of putting the failures of the last 3½ years behind us," 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 must now fire up conservatives who still doubt him while persuading swing voters that he can do a better job fixing the nation's struggling economy than Obama.

In Obama, he will face a well-funded candidate with a proven campaign team in an election that will be heavily influenced by the economy.

"It's these economic indicators that will more or less trump any good or bad that Romney potentially got out of primary season," said Josh Putnam, an assistant political science professor at Davidson College who writes the political blog Frontloading HQ.

Republicans won't officially nominate Romney until late August at the Republican national convention in Tampa, Fla. He entered the Texas primary with 1,086 convention delegates — 58 shy of the 1,144 needed to win the nomination.

Santorum, Gingrich dropped out of race weeks ago

Romney, 65, clinched the presidential nomination later in the calendar than any recent Republican candidate — but not quite as late as Obama in 2008. Obama clinched the Democratic nomination on June 3, 2008, at the end of an epic primary battle with Hillary Rodham Clinton. Four years ago, John McCain reached the threshold on March 4, after Romney had dropped out of the race about a month earlier.

Romney has been in general-election mode for weeks, raising money and focusing on Obama, largely ignoring the primaries since his competitors dropped out or stopped campaigning. His chief rivals Rick Santorum and Newt Gingrich suspended their campaigns in April and early May respectively.

Both initially offered tepid endorsements of Romney, but on Sunday Gingrich gave a full-throated defence of Romney's campaign, saying on NBC's Meet the Press that he was "totally committed to Romney's election."

Texas congressman 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.

Rich Galen, a Republican strategist who has been unaligned in the 2012 race, said the long, sometimes nasty primary fight should help Romney fine-tune his campaign organization so it can operate effectively in the general election. Galen doesn't, however, think it was relevant in toughening up Romney for the battle against Obama.

"Romney's been running for president for six years. He is as good a candidate as he's ever going to be," Galen said. "Whatever you say about him, he was better than everybody else in the race."