Possible Clues:

• Rep.'s opposite
• Party person, informally
• Republican's opponent: Abbr.
• Barack Obama, e.g.: Abbr.
• Party member, briefly
• Rep's rival
• Sen. Byrd, for one
• Jackson was the first to become pres.
• J.F.K. or F.D.R.
• Like 11-Down: Abbr.