The harshest and most antagonistic phase of the Republican presidential race began in earnest on Friday as the candidates departed the debate hall for the campaign trail, leaving behind any pretense of good will that might have remained.
Donald J. Trump, whose rivalry with Senator Ted Cruz finally erupted onstage Thursday into a bitterly personal spat over each other's values and constitutional eligibility to be president, captured this new and more unpredictable phase of the campaign, lamenting, "I guess the bromance is over."
A Republican contest that has been more national in scope than any in recent history - taking shape in contentious exchanges on the debate stage, in cable television interviews and through taunts and insults on Mr. Trump's Twitter feed - will now narrow into a more local and intimate contest as the candidates focus intensely on the first caucuses in Iowa and the first primary in New Hampshire on Feb. 9.