Better voter file data from the DNC made a huge difference in the final days of the campaign, said Mark Sullivan, founder of Voter Activation Network (VAN), in Cambridge, Mass., which makes voter file tools that are used by Democrats and other progressive groups.

"There were vast improvements in phone number quality, address quality, and large amounts of consumer data," Sullivan said

While it's unclear whether the DNC data warehouse was a deciding factor in any race, Davis and others cite at least one example of where it came into play: The Florida State Democratic party's efforts to target "Snow Bird" voters from New York and New England states were a direct product of having more detailed voter information with multiple addresses.

"They were able to communicate with Florida voters earlier in the cycle and not wait until people came back in October," said Davis. They could search for people who had addresses in northern states. They never would have been able to do that before," he said.

"[The DNC] put people in areas where we didn't expect big battles to take place, and gave them the tools and [voter] modeling data," said Sullivan. "That played no small part in what happened, especially in some of those states that weren't on the radar."