Associated Press/Paul Vernon
TCN, a company that provides political phone banking services, has a number of conservative clients in addition to pro-Clinton ones, the firm's chief technology officer Jesse Bird told Wired.
"The ironic thing is that they were probably impacting Republican calls just as much as Democrat calls," Bird said.
Workers at NextGen Climate, a pro-Clinton super PAC, reportedly began noticing problems with their dialer software Monday morning, the final day of its get-out-the-vote campaign, the Verge reported.
According to NextGen's press secretary Suzanne Henkels, the attack intermittently slowed their dialer program throughout the day, and occasionally went down for hours at a time.
The attacks may have been linked to a 4chan user by the name Sparky, who announced imminent distributed denial-of-service attacks - which slow or shut down systems by overloading them with connection requests - on pro-Clinton services and asked for a list of potential targets.
"Clinton's current Nevada phone lines are hosted on tcnp3 … all phone lines taken out," Sparky posted on 4chan, although the archive link has been taken down.
"List targets here that if taken out could harm Clinton's chances of winning and I will pounce on them like a wild animal. Not sleeping until after this election is over."
Bird, TCN's CTO, said that although the attack didn't shut down the company's services, it made things "difficult."
"They found the spot where they might be able to hurt us, and they pushed hard," he said.
Hackers also attempted to hit both Trump and Clinton's official campaign websites with similar denial-of-service attacks on Monday, but no outages were reported on either site, according to the security firm Flashpoint.