He is the last man standing in the Republican presidential race.
But presumptive nominee Donald Trump has split the party, with some senior figures declining to give him their endorsement.
The top elected Republican, Paul Ryan, said on Thursday that he was not ready to back Trump.
Ryan, the speaker of the US House of Representatives, said conservatives wanted to know if Trump shares their values.
“I hope to support our nominee, I hope to support his candidacy fully,” Ryan said on CNN.
“At this point, I’m just not there right now.”
Ryan, who was the running mate of Republican 2012 presidential nominee Mitt Romney, a harsh Trump critic, has repeatedly denied interest in running for president this year despite attempts to draft him by some in his party.
Trump, who has built a huge following with an anti-establishment message, shot back at Ryan in a statement.
“I am not ready to support Speaker Ryan’s agenda. Perhaps in the future we can work together and come to an agreement about what is best for the American people,” he said.
The Republican National Committee, under pressure to unify the party or face an electoral rout in the November 8 election, said Ryan and Trump were expected to meet soon.
It added that “only a united Republican Party will be able to beat Hillary Clinton”.
Trump’s last remaining rivals in the Republican race, US Senator Ted Cruz and Ohio Governor John Kasich, dropped out this week, clearing the New York billionaire’s path to be picked as the presidential nominee.
Despite the divisions in his party, Trump, speaking to thousands at a rally in Charleston, West Virginia, on Thursday night, was like the cat that got the cream.
“I actually wish the primaries were not over,” he told supporters.
“It is no fun this way. I want the primaries to keep going. But everybody’s out. I’m the only one left. That is okay, right?”
It is not okay for some senior Republicans.
Trump, who defeated Jeb Bush on the primary trail, is not being supported by the two former presidents produced by the Bush family – nor by Mitt Romney.
Trump’s forthright comments about women and immigrants have also been condemned by the Democrats frontrunner Hillary Clinton, still facing other battles as well as her fight for the White House.
US media is reporting that the FBI is likely to interview her in the next few weeks about her use of a private email server while she was US secretary of state and have already talked to some of her aides.