At least four Republicans say they may vote against their party on the motion to proceed, underscoring the opposition to McConnell’s bill.

The defectors include centrist Sen. Susan Collins (Maine), who panned the bill on Twitter Monday evening; fellow moderate Sen. Dean Heller (R-Nev.); and two conservatives, Sens. Rand Paul (R-Ky.) and Ron Johnson (R-Wis.).

McConnell can only afford two defections.

A loss on the procedural vote would certainly end work on the measure this week, and it could be a brutal blow to getting the legislation through the Senate on a later timeframe.