Top national security officials vowed Thursday to defend American elections against what they called real threats from Russia only weeks after President Trump seemed to accept President Vladimir V. Putin’s denials of interference during a summit meeting in Finland.

After the meeting, Mr. Trump said he had not meant to endorse Mr. Putin’s denial of election meddling, but insisted that the culprit behind the intrusion“could be other people.” A few days later, he asserted that the idea of any meddling by Russia was “all a big hoax.”

But the men and women charged with detecting and defending against any threats to the American political process showed no such ambivalence. They bluntly said that Russia was behind a “pervasive” campaign to weaken America’s democracy and influence the 2018 election.

They also sought to reassure voters that federal, state and local governments were taking steps to guard against what Christopher A. Wray, the F.B.I. director, described as a “24-7 365-days-a-year” effort by Russia to sow division as Americans head to the polls in the fall.