Several Senate votes are scheduled to test whether Republicans will allow simple-majority confirmations of a handful of President Barack Obama's long-stalled nominations. Majority Leader Harry Reid proposes to end the ability of 41 senators in the 100-member chamber to block action on White House nominations other than judges.
Some Democrats fear Republicans will retaliate if they become the majority in a future election.
A rare, three-hour private meeting of nearly all 100 senators Monday night failed to resolve the tensions. Some senators held out hope for a breakthrough early Tuesday.