WASHINGTON — Heirs who come into big money after the federal estate tax expires at the end of the month might want to hold off spending it.

The Senate rejected a bill Wednesday to extend the estate tax for two months while lawmakers work on a more permanent solution. Majority Democrats, however, vowed to come back from their holiday break early next year and pass an extension, and several key lawmakers said they would make the tax retroactive to the start of the year.

The uncertainty is already causing problems for tax planners.

"You certainly have clients who may have a few months to live, and they need to get their affairs in order," said Charles Schultz, director of private wealth and tax advisory services at the accounting firm RSM McGladrey. "It makes it difficult to plan."

This year, the inheritance tax is 45 percent on estates larger than $3.5 million. Estates smaller than $3.5 million are exempt from the tax, and married couples, with a little estate planning, can exempt a total of $7 million. That leaves less than 1 percent of all estates subject to the tax.

Under current law, the federal estate tax is scheduled to temporarily disappear next year before returning in 2011 at an even higher 55 percent rate. However, during the year without an estate tax, many estates would be subject to a 15 percent capital gains tax that they now avoid.

Sen. Max Baucus, D-Mont., called it "the yo-yo effect."

"It is an outrage," Baucus said, "that the Congress allows estate taxes to change so much."

Senate Republican Leader Mitch McConnell of Kentucky wants to extend the tax at a lower rate that would exempt more estates.