Washington ? As the House opened debate Tuesday night on a bill to overhaul election finance laws, sponsors made a few modest concessions in hopes of luring the handful of wavering members whose votes today might assure the biggest changes in campaign rules since the 1970s.

Neither side was willing to predict the outcome of tonight’s showdown votes. But backers of the bill  which would ban the brand of “soft money” donations that have pumped hundreds of millions of dollars into recent campaigns  seemed to gather momentum as night fell.

House Republican leaders, anxious to maintain a financing system that has served their party well, betrayed “an air of resignation,” one GOP strategist said. They vowed to fight, however, and readied a series of amendments that might weaken or scuttle the bill.

The measure, authored by Reps. Christopher Shays, R-Conn., and Martin Meehan, D-Mass., would ban the unlimited “soft-money” contributions that unions, corporations and wealthy individuals now give to national parties

In a compromise, Shays-Meehan supporters agreed Tuesday that the measure would take effect in 2003, leaving this fall’s congressional elections under current rules.

“We just didn’t know how to make it work” for the 2002 elections, Shays told reporters.