The pivotal vote Thursday morning to cut off debate will require a two-thirds majority to prevail. Conservative taxpayer groups as well as Republican Whip Trent Lott of Mississippi have joined in the effort to block this motion. And Sen. John McCain (R., Ariz.) will return from his presidential campaign for a press conference Thursday morning with Sens. DeMint, Coburn and other opponents.

But Democrats remain united, and Mr. Lott conceded that it was unlikely that the bill can be defeated. "It would take courage," Mr. Lott said, "Because your headline would be 'Republicans Defeat Lobbying-Ethics Reform.' "

Mr. McConnell's stance, confirmed by an aide Wednesday, is important given the fact that not just Mr. Lott, his second in command, but other Republican leadership allies, such as Sen., Robert Bennett of Utah, oppose the bill. Up for re-election in Kentucky next year, Mr. McConnell is sensitive to the political punch behind the ethics package when many voters are unhappy with the conduct of Congress. And his decision should make it easier for wavering Republicans to back the bill.