President Barack Obama has called Mitt Romney to congratulate him on winning the Republican nomination.
The president told Mr Romney "he looked forward to an important and healthy debate about America's future," Mr Obama campaign spokesman Ben LaBolt said.
Mr Romney's campaign said the call was "brief and cordial". Both men wished each other's families well.
Mr Romney surpassed the 1,144 delegates needed to win the Republican nomination by winning 105 delegates in the Texas primary yesterday.
He hopes to present himself as a worthy replacement for Mr Obama who can help revitalise a slow-moving economy, the most important issue for voters.
"Not a lot of people focus time on the characteristics of a new candidate like myself, and people will get to know me better," Mr Romney said in a Fox News interview broadcast today.
"My guess is they're going to get to know more about me than they'd like to by the time we're finished."
But his victory was partially overshadowed by his fundraising appearance in Las Vegas with real estate baron Donald Trump, who continues to push the false allegation that Mr Obama was born outside the United States and is ineligible to hold the presidency.
The former Massachusetts governor's ties to Mr Trump, a boisterous voice in the Republican party, mark the latest example of hugely wealthy Mr Romney's less than sure-footed run for the presidency.
Mr Romney explains his readiness to accept Mr Trump's financial help by saying he is not required to agree with all the positions held by his backers. Mr Romney contends that Mr Obama is born in the US, despite the claims by Mr Trump and other extreme Republicans.
The campaigns of both Mr Obama and Mr Romney are expected to raise vast sums - perhaps as much as a billion dollars each - in what will be the most lavish spending ever in a US presidential campaign.