Washington (CNN) - The Senate is expected to hold a procedural vote Tuesday on whether to end debate on a campaign finance bill, and Democrats fear a unified Republican filibuster will prevent the measure from moving to a final vote.
A key Republican, moderate Sen. Susan Collins of Maine, opposes the bill because it puts less restrictions on labor unions - a key Democratic constituency - than on corporations and other large donors, an aide to Collins said Monday.
"The bill would provide a clear and unfair advantage to unions, while either shutting other organizations out of the election process or subjecting them to onerous reporting requirements that would not apply to unions," said the Collins aide, Kevin Kelley.
Collins is one of the few Republican senators considered a possible vote in support of Democratic-backed proposals. With Republicans holding enough Senate seats to block legislation, the inability of Democrats to attract support from Collins shows the precarious status of the bill.