The House speaker, Paul D. Ryan, dealt a hammer blow to Donald J. Trump’s presidential candidacy on Monday, dashing any remaining semblance of Republican unity and inviting fierce backlash from his own caucus by announcing that he would no longer defend Mr. Trump.

Mr. Ryan’s stance drew an immediate rebuke from Mr. Trump, who posted on Twitter that Mr. Ryan should focus on governing “and not waste his time on fighting Republican nominee.”

Mr. Ryan informed Republican lawmakers on a morning conference call that he would never again campaign for Mr. 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 the condition of anonymity.

Effectively conceding defeat for his party in the presidential race, Mr. Ryan said his most urgent task was ensuring that Hillary Clinton did not take the helm with Democratic control of the House and Senate, two lawmakers said.