President Barack Obama has called Mitt¹ Romney, his challenger in November's election, to congratulate him on winning the Republican nomination².

奥巴马总统致电米特·罗姆尼——十一月大选中他的主要对手——祝贺他赢得共和党提名。

The rival campaigns exchanged well wishes, a day after Mr Romney's easy victory in the Texas primary.

But his appearance alongside Donald Trump³, who has been reviving long-discredited doubts about Mr Obama's US citizenship⁴, stirred controversy⁵.

Mr Romney is the first Mormon candidate to contest a presidential election.

'Brief and cordial'

On Tuesday, he amassed⁶ the majority of 1,144 delegates which is needed to be anointed as the Republican nominee⁷ at the party convention in Tampa, Florida in late August.

Obama campaign spokesman Ben LaBolt said on Wednesday that the president had called Mr Romney to congratulate him on securing the Republican nomination.

"President Obama said that he looked forward to an important and healthy debate about America's future, and wished Governor Romney and his family well throughout the upcoming campaign," said Mr LaBolt.

A Romney campaign spokesman said that the call had been "brief and cordial（热忱的） ".

"Governor Romney thanked the president for his congratulations and wished him and his family well," the official said.

Mr Romney, who has been the presumptive nominee for several weeks as his rivals withdrew, said in a statement on Tuesday that he was "honoured" and "humbled⁸" to have wrapped up the Republican race.

As the Texas primary results came in, the former Massachusetts governor was attending a Las Vegas fundraiser with reality television personality Mr Trump, whose popularity among conservative Republicans prompted him to briefly⁹ flirt¹⁰ with a presidential run himself this year.

Although Mr Trump did not mention the "birther" conspiracy¹¹ theory at Tuesday evening's event, he told CNBC hours earlier: "A lot of people are questioning [Mr Obama's Hawaiian] birth certificate. They're questioning the authenticity¹² of his birth certificate."