'I'm just not ready' to back Donald Trump, says GOP house speaker

Republican House speaker Paul Ryan has said that he couldn't lend his support to Donald Trump, the party's presumptive presidential nominee.

"I'm just not ready to do that at this point. I'm not there right now," he told CNN's "The Lead with Jake Tapper" in an interview.

The Wisconsin Republican's rejection makes him the highest level GOP official to reject Trump since the New York billionaire became the only candidate standing in the party's nominating contest.

Ryan's move gives down-ballot Republicans cover to hold off on supporting the real estate mogul.

Ryan said eventually hoped backing Trump and "to be a part of this unifying process" but the first move must come from the latter.

He said that he wanted Trump to unify "all wings of the Republican Party and the conservative movement" and then run a campaign that would allow Americans in being part of something they that they are proud to support.

The House speaker, who will be chairman of the Republican national convention in July in Cleveland, hoped to come around in supporting Trump.

However, neither of the last two Republican presidents , George H.W. Bush and George W. Bush will attend the GOP convention in Cleveland.

Nor will the 2008 nominee, John McCain, or the 2012 nominee, Mitt Romney be present.

Ryan said that he had only started considering whether he would support Trump after the real estate mogul won Indiana primary on Tuesday, knocking both Texas Senator Ted Cruz and Ohio Governor John Kasich out of the race.

"I thought about this two days ago. I thought, actually, this thing was going to go to June 7 at the very least -- probably to a convention -- and so this is all pretty new for us," he said.

"The bulk of the burden on unifying the party will have to come from our presumptive nominee," Ryan said.

The House Speaker pressed that the party hopes the nominee aspires to be Lincoln.

" I don't want to underplay what he accomplished. ... But he also inherits something very special, that's very special to a lot of us. This is the party of Lincoln and Reagan and Jack Kemp. And we don't always nominate a Lincoln or a Reagan every four years, but we hope that our nominee aspires to be Lincoln- or Reagan-esque -- that that person advances the principles of our party and appeals to a wide, vast majority of Americans," he said.

"And so, I think what is necessary to make this work, for this to unify, is to actually take our principles and advance them. And that's what we want to see. Saying we're unified doesn't in and of itself unify us, but actually taking the principles that we all believe in, showing that there's a dedication to those, and running a principled campaign that Republicans can be proud about and that can actually appeal to a majority of Americans -- that, to me, is what it takes to unify this party," he said.