James Clapper, director of the Office of National Intelligence, said he expects more attempted hacks as the campaigns intensify.
This would follow a pattern established in the last two presidential elections.
The FBI is working with the campaigns to make their networks more secure.
The Department of Homeland Security is also assisting, but cyber-security experts said political campaigns have not done much to improve their defences since 2008.
Hacking was widespread during the 2008 election cycle. The Office of National Intelligence described its scale as "like no other" in a report released earlier this month.
V Newtown Miller, a data security consultant advising government agencies, said the hackers' attempts could have a huge effect on presidential politics.
"It's a matter of when and how serious of an impact it is going to have on this election," said Mr Miller, who believes these foreign hackers attempt to extract sensitive information, rather than commit cyber vandalism.
If a hacker is able to reveal embarrassing information about a candidate, it could sway how people vote in the election.
But simply taking down a candidates website for a few hours could also have an effect, as it limits the campaign's ability to online fundraise, as happened to Mitt Romney in 2012 for several hours.
The global hacking collective, Anonymous, declared a cyber war against Republican candidate Donald Trump several weeks ago.
They are encouraging their members to target Mr Trump's business interests as well as his campaign resources.
In 2008, hackers thought to be working for the Chinese government obtained a letter by Senator John McCain expressing support for Taiwan.
A Chinese diplomat called the McCain campaign to complain about the letter before it had been sent.
Former Secretary of State Hillary Clinton has drawn criticism for operating a private email server during her time as the nation's top diplomat.
She is being investigated by the FBI to determine whether classified information was sent through the unsecured server.
The US intelligence chief has said cyber-hackers working for foreign governments are targeting the candidates in this year's presidential election.