But Clapper also said the Russian hacking efforts "did not change the vote tallies" in the election, which ended with Donald Trump's victory. Clapper was referring specifically to the question of whether voting machines could have been manipulated to affect the outcome.
Clapper added that when it comes to the the question of whether Russian hacking affected "the choices that the electorate made ... there's no way for us to gauge that."
"Russia clearly has assumed an even more aggressive cyber posture," Clapper said.
He said later: "They do pose an existential threat to the United States."
Clapper told the committee that a forthcoming intelligence report "will ascribe a motivation" by Russian President Vladimir Putin for the hacking. But he said he did not want to "pre-empt" the report by revealing the suspected motivation.
President Barack Obama has been briefed on the report, a senior intelligence official told NBC News. Obama last week ordered a series of retaliatory steps against Russia for the hacking.
During the hearing, the price of VanEck Vectors Russia, an exchange-traded fund that seeks to replicate the price of the DAXglobal Russia+ Index of larger Russian companies, fell more than 1.5 percent. The decline put the ETF on pace for its first negative day since Dec 19, when RSX lost 2.23 percent.
Questions about Russian interference have swirled furiously since Trump's victory over Democrat Hillary Clinton in November.
While the intelligence community believes top Russian officials were behind cyberattacks aimed at Clinton, Trump has been dismissive of those claims. Trump's backers also have seen the claims as an attack on the legitimacy of his election, but some Republicans have insisted that the matter must be thoroughly investigated.
"Every American should be alarmed by Russia's attacks on our nation," committee Chairman Sen. John McCain, R-Ariz., said at the start of the hearing.
"There is no national security interest more vital to the United States of America than the ability to hold free and fair elections without foreign interference."