President Barack Obama has called Mitt Romney, his challenger in November's election, to congratulate him on winning the Republican nomination.
The rival campaigns exchanged well wishes, a day after Mr Romney's easy victory in the Texas primary.
But his appearance alongside Donald Trump, who has been reviving long-discredited doubts about Mr Obama's US citizenship, stirred controversy.
Mr Romney is the first Mormon candidate to contest the presidential election.
'Brief and cordial'
On Tuesday, he amassed the majority of 1,144 delegates which is needed to be anointed as the Republican nominee 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".
"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 "humbled" 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 briefly flirt with a presidential run himself this year.
Although Mr Trump did not mention the "birther" conspiracy 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 authenticity of his birth certificate."
Romney poll boost
Mr Romney has not repudiated Mr Trump's comments, though his aides have said the GOP nominee believes the president was born in the US.
The swing voters who are expected to hold the key to victory in November's presidential election are likely to look askance at Mr Trump's rhetoric, correspondents say.
With opinion polls suggesting an extremely close race between Mr Romney and the Democratic president, the Republican candidate is holding a major fundraising event every day this week in the state of California.
A Washington Post-ABC News poll released on Wednesday suggested that Mr Romney's appeal is rising, with 41% of respondents saying they had a positive impression of him, while 52% said the same of Mr Obama.
An earlier poll showed Mr Obama had a 52% rating against 35% for Mr Romney.
The Obama and Romney campaigns are expected to raise huge sums - perhaps as much as $1bn (£643m) each - in what may be the most lavish spending ever in a US presidential campaign.
Meanwhile, a spelling error embarrassed the Romney campaign as it released a photographic app that said "A Better Amercia".
The "With Mitt" application allows users to show their support by personalising a photo with an overlaid Romney slogan, but the typographic howler went viral on social networking websites.
As Mr Romney achieved his delegate majority, the Democratic National Committee released a video titled Mitt Romney: Little to Like.
The video highlights the Republican candidate's association with Mr Trump and also suggests that Mr Romney favours the wealthy.
This is Mr Romney's second bid for the White House, after he ran unsuccessfully in 2008.
His father, George Romney, a former Michigan governor, also ran unsuccessfully for the Republican nomination in 1968.
Political pundits are now speculating who Mr Romney will pick as his vice-presidential running mate.