The DNC was alerted in the early hours of Tuesday morning by a cloud service provider and a security research firm that a fake login page had been created in an attempt to gather usernames and passwords that would allow access to the party's database, the source said.

The DNC and the two companies involved in detecting the operation say they believe they thwarted a potential attack.

The fraudulent page was designed to look like the access page Democratic Party officials and campaigns across the country use to log into a service called Votebuilder, which hosts the database, the source said, adding the DNC believed it was designed to trick people into handing over their login details.

The DNC is investigating who may have been responsible for the attempted attack, but that it has no reason to believe its voter file was accessed or altered.

The fraudulent page was hosted on a cloud computing platform called DigitalOcean, which acted to remove the page as soon as it was alerted by Lookout, the cloud company said.

"We see no evidence that any sensitive data was stolen and our initial investigation indicates that we were able to address this threat prior to the attack being launched", Josh Feinblum, chief security officer at DigitalOcean, said in a statement.