The Democratic official described the latest attempt as a sophisticated phishing operation in which a hacker used a fake login page in an attempt to secure passwords, the presumed objective being to access the voter files.
Additionally Democratic Sen. Claire McCaskill, who is one of the toughest reelection's contest of this cycle, said her campaign was unsuccessfully targeted by Russian hackers past year.
The initial detection of the apparent hacking attempt by a cybersecurity firm on Monday prompted the DNC to call the Federal Bureau of Investigation, the source said. "It was very convincing", he said.
A third party "not authorized by the DNC or its vendors" set up the site to test Democrats' resiliency to phishing attacks.
"We, along with the partners who reported the site, now believe it was built by a third party as part of a simulated phishing test on VoteBuilder", Bob Lord, the chief security officer who was previously Yahoo's chief information security officer, said in a statement late Wednesday.
The DNC learned of the attack on Tuesday when a cloud service provider and security research firm noticed a fake login page that was being used to collect usernames and passwords. "It is their responsibility to protect our democracy from these types of attacks".
"These threats are serious and that's why it's critical that we all work together, but we can't do this alone", Lord said to CNN.
The chairman of the Michigan Democratic Party, Brandon Dillon, did not respond to a request for comment.
That disclosure, coming less than three months ahead of the 2018 midterms, demonstrated Russia's continued efforts to destabilize United States institutions.
President Donald Trump has dismissed as a "witch hunt" Mueller's continuing investigation into Russian interference and whether anyone close to Trump colluded in it.
And the party's call to bring in federal agents indicates how concerned officials are about any attempts to hack information ahead of the coming midterm elections.