Mitt Romney clinches nomination in Texas primary
Mitt Romney has secured his place as the Republican challenger to Barack Obama in November's US presidential election, following a primary in Texas.
Results show he easily cleared the benchmark of 1,144 delegates to secure the nomination.
Mr Romney said he was "humbled". He is the first Mormon from a major party to contest the presidential election.
He is set to be officially anointed as the Republican nominee at the party convention in Florida in late August.
Mr Romney was attending a private fundraiser in Las Vegas with real estate tycoon Donald Trump as results of the Texas primary emerged on Tuesday night.
He will spend the coming days at a series of West Coast events to raise money as the race against Barack Obama is set to intensify.
Mr Romney has been the presumptive nominee for several weeks as his rivals withdrew or suspended campaigning.
With 1,086 delegates secured as he entered the Texas race, Mr Romney only needed 58 to pass the 1,144 mark, and he won 69% of the vote. Texas awards 152 delegates proportionally.
He is set to surpass his father, George Romney, a former Michigan governor who ran unsuccessfully for the Republican nomination in 1968.
Opinion polls suggest Mitt Romney is locked in an extremely close race with Mr Obama in November.
"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," Mr Romney said in statement.
As Mr Romney achieved his delegate majority, the Democratic National Committee released a video titled Mitt Romney: Little to Like.
The video suggests Mr Romney favours the wealthy, and it highlights the former governor's association with Mr Trump, who has been touting a long discredited theory that the president was born in Kenya.
On Monday the Romney campaign released an attack ad criticising Mr Obama over a federal loan to a solar panel maker which later went bankrupt.
On Tuesday Mr Romney spent the day campaigning in Colorado and Nevada. He also met casino tycoon Sheldon Adelson, who gave millions to supporters of Newt Gingrich's campaign during the primary season.
Experts say the outcome of November's election could depend on what happens in a handful of battleground states, including Colorado and Nevada, as the candidates vie for the support of independent voters.
Political pundits are now speculating who Mr Romney will pick as his vice-presidential running mate.