Senate Majority Leader Harry Reid (D-Nev.) has agreed to allow amendments on legislation extending several tax measures that expired last year, ending Republican procedural roadblocks on the legislation.

Around 2 p.m. on Monday, the Senate is expected to turn its attention to the House bill that extends several tax breaks that expired last year. Senate Finance Committee Chairman Max Baucus (D-Mont.) will offer a substitute amendment that adds several spending measures to the bill, including an extension for highway spending. Debate over amendments to the bill is expected to last through most of the week.

Republicans agreed to move on extender legislation without an agreement on how to handle the estate tax. Senate Minority Whip Jon Kyl (R-Ariz.) made reaching an agreement on the tax a caveat to allowing a vote on extenders. 

The longer the fate of the estate tax remains unknown, the more it becomes a political liability for lawmakers. Staffers say phone calls are on the rise from constituents demanding action on the estate tax.

The estate tax is currently repealed, and barring congressional action it returns next year to pre-2001 levels by socking estates worth more than $1 million with a tax that tops out at 55 percent.

Republicans, and arguably more than a few Democrats, oppose this level and prefer rates reminiscent of 2009, when estates worth over $3.5 million were taxed at a top rate of 45 percent. Kyl seeks to cap the tax at 35 percent on estates worth more than $10 million per couple. But either option creates a constitutional dilemma if lawmakers make the tax retroactive to Jan. 1.

The move could create endless litigation as taxpayers cry foul over being taxed subsequent to receiving their inheritance. The longer Congress postpones its decision on the estate tax, the more estates could be subjected to a retroactive tax that could incite more lawsuits.