House Speaker Paul D. Ryan dealt a hammer blow to Donald J. Trump’s presidential candidacy Monday, dashing any remaining semblance of party unity and inviting fierce backlash from his own caucus by announcing that he would no longer defend Trump’s candidacy. Mr. Ryan’s stance drew an immediate rebuke from Trump, who posted on Twitter that Ryan should focus on governing instead of feuding with him. But Ryan informed Republican lawmakers on a morning conference call that he would never again campaign alongside Trump and would dedicate himself instead to defending the party’s majority in Congress, according to five lawmakers who participated in the call and spoke on condition of anonymity.