US House Speaker Paul Ryan has said he cannot currently support Donald Trump as Republican presidential nominee.
Mr Ryan, the highest-ranked elected Republican, called for Mr Trump to strive for “higher aspirations”.
Mr Trump, he said, “deserved” the nomination but must do more to unite the party.
Former presidents George W Bush and George H W Bush also said on Thursday they will not endorse the controversial presumptive nominee.
“I think what a lot of Republicans want to see is that we have a standard bearer that bears our standards,” Mr Ryan told CNN.
There has been persistent speculation that Mr Ryan, the 2012 vice-presidential nominee alongside Mitt Romney, may make a last-minute run for the top job.
But he has frequently sought to dispel those claims and did so again on Thursday.
In December 2015, Mr Ryan harshly criticised Mr Trump’s proposal to ban Muslims from entering the US.
He said it was “not what this party stands for and more importantly it’s not what this country stands for”.
Although several senior Republicans like Mr Romney have already voiced their objection to the nominee, Mr Ryan is the highest ranked to do so.
Protests have plagued appearances by Mr Trump, with particular focus on his plan to build a wall on the Mexican border and deport 11 million undocumented immigrants.