The death of Senator Robert C. Byrd of West Virginia threw into doubt the ability of Democrats to win approval this week of a financial regulation bill and underscored how the smallest changes in the size and composition of their Congressional majority have complicated their efforts to pass ambitious legislation over near-unanimous Republican opposition…

It was the second time in recent months that the loss of a longtime colleague threatened the ability of Democrats to advance a top legislative priority. The death last year of Senator Edward M. Kennedy of Massachusetts — who was second only to Mr. Byrd in length of Senate service — and his replacement in January by a Republican, Senator Scott Brown, forced Democrats into procedural gymnastics to enact the health care law.

Democrats now need to reverse some internal opposition and hold on to a trio of Republicans to push through the Wall Street measure that was agreed to by House and Senate negotiators last week. The House is scheduled to press ahead with the legislation this week.