“Why wasn’t it more important to tell the American people the length and breadth of what the Russians were doing to interfere in an election than any risk that it might be seen as putting your hand on the scale?” Mr. Schiff asked. “Didn’t the public have a compelling need to know?”

Asked why former President Barack Obama did not make his own announcement that a foreign power was meddling in the election process, Mr. Johnson suggested administration officials believed just his involvement would inherently politicize the facts.

“We were very concerned that we not be perceived as taking sides in the election, injecting ourselves into a very heated campaign or taking steps to delegitimize the election process and undermine the integrity of the election process,” he said.

Noting that the hacking happened “at the direction of Vladimir Putin himself,” Mr. Johnson said he was moved to try to shield the nation’s election system by the “unprecedented” nature of Russian interference in the last election.

“What I mean is that we not only saw infiltrations, but we saw efforts to dump information into the public space for the purpose of influencing the ongoing campaign,” he said, referring to the disclosure of hacked emails.

Republicans also seized on the statement in January by James B. Comey, the former F.B.I. director, that the D.N.C. refused to turn over its servers to investigators after it was discovered that the servers had been hacked.