Speaker Paul D. Ryan, the nation’s highest-ranking elected Republican, said Thursday he was “not ready” to endorse Donald J. Trump, the presumptive Republican nominee. Mr. Ryan, who is also chairman of the Republican National Convention, has repeatedly said he would support his party’s nominee. But he has carved out some terrain to differentiate himself, by insisting that House Republicans write their own policy agenda this year and by giving speeches that at times criticized Mr. Trump’s views. Mr. Ryan said Republicans needed a “standard-bearer” who embodied the party’s standards. “I don’t want to underplay what he accomplished,” Mr. Ryan said...