Ad firms that serve political campaigns bolster online video offerings.
Video ads reached 42 percent of users over the course of the month.
Despite video monetization goals, Google serves fewer video ads than competitors.
Tremor Media, ADAP.TV, and BrightRoll served substantially more ads in December than they did the previous month, according to comScore.
Merged entity will be second largest online video ad company in the U.S. behind Hulu, says comScore.
ComScore estimates 45.4 percent of users viewed at least one video ad over the course of the month.