Let's ask Google.

  "democratic senators"
  > About 969,000 results (0.52 seconds)

  "democrat senators"
  > About 92,800 results (0.42 seconds)

Bonus:

  "democratic senators" site:breitbart.com
  > About 1,010 results (0.32 seconds)

  "democrat senators" site:breitbart.com
  > About 2,300 results (0.27 seconds)

Conclusion: "democratic" is significantly more common outside of right-wing propaganda.