The Senate bill, approved just before Congress departed for the Memorial Day recess, contains a provision that would require illegal immigrants to pay back taxes in order to start on a path toward naturalization.

Revenue measures are supposed to originate in the House, according to the Constitution. A House member in the conference committee could declare that the Senate bill is unconstitutional, squelching negotiations to reconcile House and Senate immigration legislation.

Traditionally, the chairman of the House Ways and Means Committee, currently Rep. William Thomas, R-California, would point out the problem and "blue slip," or kill, the Senate bill. Thomas has not commented publicly on whether he intends to do so.

In order to remedy the constitutional technicality, Senate Majority Leader Bill Frist, R-Tennessee, has proposed that the Senate take a tax bill previously approved by the House and add the Senate language on immigrant back taxes.

That measure would be approved by unanimous consent in the Senate and go to the House, where the House immigration bill would be attached. Then the House would vote to proceed to conference.

A Frist aide asserts that Senate Minority Leader Harry Reid, D-Nevada, is insisting that the Senate and House go directly to the conference committee with the bills as each chamber has approved them.

But a spokeswoman for Frist says that taking that route would mean "the bill is dead" because a House member would declare it unconstitutional.

"We’re at a standstill," says Carolyn Weyforth, Frist’s press secretary. For Democrats, "this is a campaign issue. They don’t want to address (immigration) until after the election."

On June 6, Reid indicated that he fears that attaching the Senate immigration bill to a House tax measure would open the flood gates for tax policy changes in conference. He said that he would drop his objections if Republicans assure him that they won't use the immigration bill as a vehicle for non-immigration tax reform, according to published reports. He also said that it is up to Bush to prevent House Republicans from "blue-slipping" the Senate immigration bill.

In December, the House approved an immigration bill that focuses on border security and workplace enforcement. The Senate passed a comprehensive bill that contains enforcement provisions as well as a guest worker program and a path to naturalization for most of the country’s approximately 11 million illegal immigrants. The bills must be combined into a final measure that would be voted on again in each house.

"If we can get to conference, differences can be worked out," Weyforth says.

--Mark Schoeff Jr.