WASHINGTON — After a month of speculation and pleas ranging from the comic to the mildly desperate, Speaker Paul D. Ryan held an unusually formal news conference Tuesday afternoon to rule out once and for all, he said, his candidacy for the Republican nomination for president. “Let me be clear,” Mr. Ryan said. “I do not want nor will I accept the nomination of our party.” He added that he had a message for convention delegates: “If no candidate has the majority on the first ballot, I believe you should only turn to a person who has participated in the...