Hartke and Brock were both facing tough reelection challenges that year, so I'll put their colleagues from their respective states on the ticket instead. The conservative Buckley runs with the moderate conservative Baker, and the moderate liberal Proxmire runs with the more liberal Bayh.
Buckley is well respected, and is helped by Baker, but is seen as too conservative by a majority. Proxmire's record as an advocate for government reform helps him win in this first post-Watergate Presidential election.
Sen. William Proxmire (D-WI) / Sen. Birch Bayh (D-IN) 365 52.2%Sen. James L. Buckley (R-NY) / Sen. Howard Baker (R-TN) 173 46.7%others 1.1%