But if written narrowly, a background check provision could potentially win bipartisan support.
"If it does fail, then Congress is going to need to explain why they thwarted the will of the electorate," Mr. Feinblatt said of the background check provision.
Representative Bill Thomas, Republican of California, the chairman of the Ways and Means Committee and author of the bill, said the check provision was not left out through malice and said senators were free to bring it up in conference.
Several Democrats have voiced opposition to the bill's "card check" provision, which would generally require employers to recognize a union as soon as a majority of employees signed cards supporting it.
The amendment offered by Senators Toomey and Manchin is set to replace the background check provision in legislation about to hit the Senate floor after the chamber voted 68-31 on Thursday to open debate on it.
This amendment would replace the background check provision of the original legislation, which would also create harsher penalties for the so-called straw purchasing of guns, in which people buy guns for others who are not able to legally.
If the Manchin-Toomey version is defeated, the gun measure on the floor would be left with a more expansive background check provision with even less support, and the overall gun bill would have an even steeper climb to passage.
program has been crucial in recent union victories in Trenton, including a law allowing government agencies to require even nonunion contractors to adhere to the terms of union contracts; one mandating paid family leave for many private-sector workers; and a "card check" provision making it possible for employees to unionize without elections.
This amendment was set to replace the background check provision of the original legislation, which would also create harsher penalties for the so-called straw purchasing of guns, in which people buy guns for others who are not able to do so legally.
The New York Times reported last week that the EFCA 's card check provision was dead, and while Stern issued a statement refuting this, it is consistent with what political insiders have been saying about the cost of labor disunity.
