WASHINGTON: 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.

“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 per cent of the vote, against 12 per cent for Ron Paul, 8 per cent for Rick Santorum and 5 per cent for Newt Gingrich, according to CNN, with 99 per cent 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.

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.

Poll aggregates show Obama narrowly ahead. The latest RealClearPolitics average shows the president with a two-point lead, 45.6 to 43.6 per cent.