"We've already had some indications of that," James Clapper, the director of national intelligence, said in Washington.
During the 2008 presidential campaign, U.S. intelligence agencies traced massive cyber attacks to China. At that time, both the Democratic candidate, now-President Barack Obama, and his Republican rival John McCain, were targeted. Officials said hackers were trying to seize sensitive data, including private emails and information on high-level economic and national security briefings senior aides might have received.
The Department of Homeland Security and the Federal Bureau of Investigation "are doing what they can do educate both campaigns against potential cyber threats," Clapper said at an event at the Bipartisan Policy Center.
He didn't offer any detail on how the intrusions were detected or name which campaigns were targeted. But he offered a prediction.
"As the campaigns intensify, we'll probably have more," Clapper said.
California Rep. Adam Schiff, the ranking Democrat on the House Intelligence committee, told CNN's Situation Room that Clapper's comments are "a reminder that every facet of our lives are dependent on computers, which are by their nature vulnerable to intrusion."
Schiff said that given the intense scrutiny paid to the 2016 campaign and the implications it has for U.S. foreign policy "it's no surprise that actors are launching cyber-attacks against presidential campaigns."
Clapper was addressing the challenge of balancing the need to protect U.S. businesses and citizens' privacy in the face of an "incredibly complex set of cyber threats."
Russians, Chinese most sophisticated
Those threats come from a dizzying array of enemies, he said, from groups like Anonymous to nation states like Iran, which has attacked U.S. banks and infiltrated the control system of a dam in New York.
North Korea's attack on Sony Pictures damaged a major U.S. corporation, while China is the suspected culprit behind the 2015 theft of millions of federal workers' security-related personal information in an attack on the Office of Personnel Management.
"The Russians and Chinese are far more sophisticated and could do real damage if so inclined. Then there are terrorist groups," Clapper said. "Each has different objectives. The one thing they have in common, they all operate on the same internet."
Clapper said in his decades-long career in intelligence, he doesn't "recall a time when we've been beset by a wider array and more diverse array of threats and crises than we are today." He puts cyber at the top of the list when he discusses risks because of the variety of intrusions that come "around the clock."
That, he said, "will likely expand" at increasing costs to U.S. businesses and economic security.
The rapid pace of change in the cyber environment cuts both ways for the intelligence community, Clapper said.
Pointing to the "internet of things" that links everything from household appliances to clothes to the web, the intelligence director said it "presents lots of vulnerabilities" but "it's probably lots of opportunities for us," too.
It also presents administrative challenges. Clapper wondered how intelligence officers would be affected when their clothes might be linked to the internet.
"Even now," he said, "I need security clearance for my hearing aids" because they use Bluetooth technology.
Future attacks won't just steal information
Clapper cited the attack on the Office of Personnel Management as an example of how damaging the hacks can be and the shape that future attacks might take.
OPM files contained rich material available in federal workers' background checks that could allow foreign agencies to target those employees, their families, co-workers or neighbors, Clapper said.
"At this point, we haven't seen any direct evidence of this that we can conclusively tie to the breach," Clapper said.
Future attacks, he predicted, won't just steal information.
"I believe we'll see more cyber operations that will change or manipulate information," Clapper said, as U.S. enemies work "to compromise its reliability" instead of deleting or simply stealing it.
Ninety percent of all cyber intrusions are initiated by successful spearfishing, or the use of fake web links, Clapper said. Foreign intelligence operations very often "can and do get access to our systems just by pretending to be someone else," he said.
Clapper added that it's even a challenge within the intelligence community to make sure employees resist the temptation to open an attachment they don't recognize.
"We still have issues with that," Clapper said. "The Chinese in particular are cleaning us out because they know we're supposed to do these things and we don't do them."
He recommended companies and individuals patch their software often, keep information segmented so that any hack into your system won't immediately mean access to all of it; stay updated on regular bulletins that DHS and the FBI put out, and learn what spearfishing looks like.
"Bad cyber actors are using precisely these avenues," he said.
Jason Grumet, president of the Bipartisan Policy Center, said that not long ago, few people imagined the possibility that private companies would have to stand up to nation states like China, which has been implicated in the theft of company information.
That's become the new reality, said Mike Rogers, former chairman of the House Intelligence Committee. He offered brokerage firm J.P. Morgan as an example, saying that this year the company will spend $600 million to protect itself against attacks from national actors.