WASHINGTON (AP) — 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. Even in an election cycle that's exposed extreme and very public divisions within the 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, 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. 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, Ryan's national finance chairman. Trump told a crowd in South Carolina in February that Ryan doomed the GOP presidential ticket four years ago by saying entitlement programs need reform.