Two seasoned dealmakers are fine-tuning a bipartisan retirement package with substantial tax breaks for employers that they plan to roll out in the lame-duck Senate session, adding another legislative vehicle to the year-end mix.

In addition to this new draft bill, retirement legislation pending includes the House-passed Family Savings Act (H.R. 6757) and languishing Retirement Enhancement and Savings Act (S. 2526), authored by retiring Senate Finance Committee Chairman Orrin Hatch (R-Utah) and Sen. Ron Wyden...