Why Paul Ryan still won't back Donald Trump

House Speaker Paul Ryan is refusing to support Donald Trump as the Republican nominee for president, insisting Thursday that the businessman must do more to unify the GOP.

The surprise declaration from Ryan on CNN's "The Lead" amounted to a stunning rebuke of Trump from the Republican Party's highest-ranking officeholder.

"I'm just not ready to do that at this point. I'm not there right now," the Wisconsin Republican said. "And I hope to. And I want to, but I think what is required is that we unify this party."

Even in an election cycle that's exposed extreme and very public divisions within a GOP, Ryan's decision to withhold his support from Trump was remarkable, as the GOP's top elected leader, second in line to the presidency, turned his back on his own party's presumptive nominee.

Ryan had maintained his silence since Trump effectively clinched the nomination with a commanding win in Indiana on Tuesday that forced his two remaining rivals from the race. Other Republican leaders, including Senate Majority Mitch McConnell, offered their grudging support for Trump, and Ryan had seemed likely to eventually do the same.

Instead he balked, in comments that could have more to do with his own political future and potential run for president in 2020.

"We will need a standard-bearer that can unify all Republicans, all conservatives, all wings of our party, and then go to the country with an appealing agenda that can be appealing to independents and disaffected Democrats," Ryan said. "And we have work to do on this front, and I think our nominee has to lead in that effort."

Ryan made clear he won't be supporting Hillary Clinton and that he wants to come around to supporting Trump

"If we don't unify all wings of this party, we're not going to win this election," he said. "So the question is, 'What can you do to unify all wings of the party to go forward?'"

Ryan himself, his party's 2012 vice presidential nominee, had been seen as a possible "white knight" candidate who could emerge as an alternative to Trump at a contested convention. He called a press conference last month to rule himself out, and Trump now looks set to gather the 1,237 delegate votes needed to clinch the nomination ahead of the July convention in Cleveland, ruling out the possibility of a contested convention.

Ryan will serve as the convention chairman, presiding over portions of the proceedings that will elevate Trump to the official status of nominee.

He's spent his time since becoming speaker last fall in part on working on a GOP agenda that members of Congress could run on regardless of who emerges as the GOP nominee.

Trump has flouted a number of conservative tenets in his campaign. He has praised Planned Parenthood even as a House GOP committee investigates its practices regarding fetal tissue collection. He's bashed trade agreements even as a major trade deal is pending before Congress. He's criticized Ryan personally for promising to cut benefit programs such as Medicare. And just Wednesday he said he was open to the idea of raising the minimum wage.

The Christian Science Monitor's Peter Grier writes that there are rumblings among disaffected Republican loyalists who are talking about mounting a third-party effort to challenge Trump. 

The real problem about this unrest among the party elite is that it’s about them as much as about real GOP voters. They’re appalled by Trump and think he’ll lead the party to defeat, but lots of rank-and-file party members don’t agree with that, and are happy with Trump. Who was their choice, after all.

With deep concerns about Trump at the top of the ticket, Ryan is positioning himself to play a central role in helping to protect vulnerable Republican House and Senate candidates heading into the general election, said Spencer Zwick, who is Ryan's national finance chairman.

"Paul Ryan is the single most effective tool and person to maintain control of the Senate and the House," said Zwick, who attended a Detroit-area fundraiser with Ryan on Wednesday.

"He's focused on an agenda. He's constantly out there talking about his agenda. Talk to (Sen. Rob) Portman in Ohio, or congressmen who are up, and they are very happy with the fact that Paul Ryan is promoting an agenda they can all sign on to. Many people aren't sure what the Trump agenda is yet."