The proposal and other key bills died without a vote late Tuesday after Republicans abruptly ended the day's work. House Speaker Frank McNulty said lawmakers had reached an impasse and civil unions would die along with bills to overhaul school discipline policies and setting a blood-level marijuana limit for drivers.
Republicans control the House but enough members of their party supported the civil unions legislation to pass it.
The GOP launched a filibuster to prevent a vote on civil unions and Democrats countered with a procedural move to try to stop debate on other bills.
Chants of "shame on you" from gay rights supporters thundered through the chamber after debate was stopped.