LAS VEGAS — President Trump said Friday that a disclosure by American intelligence officials that Russia was again meddling in a presidential election in his favor was merely another partisan campaign against him, dismissing the warning as a hoax cooked up by rivals.

“Another misinformation campaign is being launched by Democrats in Congress saying that Russia prefers me to any of the Do Nothing Democrat candidates who still have been unable to, after two weeks, count their votes in Iowa,” Mr. Trump wrote on Twitter. “Hoax number 7!”