WASHINGTON — As the Senate prepared to push through a sweeping tax package negotiated by the White House and congressional Republicans, liberal Democrats were laying plans Tuesday to unravel the deal in the House by changing the terms of a provision that would revive the federal estate tax.

Senate leaders, meanwhile, postponed a vote on the $858 billion package until this afternoon. Despite the delay, the measure was expected to sail through the Senate with even stronger support than it received in a test vote this week, when 83 senators voted to advance the package in the strongest bipartisan vote on a major initiative since President Obama took office.

The measure would extend for two years an array of tax breaks enacted during the George W. Bush administration that are set to expire Dec. 31, including benefits championed by Republicans for the wealthiest households. In return, Obama secured another year of emergency jobless benefits and fresh incentives to boost the economy, including a two-percentage-point reduction in the payroll tax for all workers in 2011.

Sen. Sherrod Brown, D-Ohio, said he was leaning toward voting for the package after voting against it Monday "to send a message to the House that there are allies here."

The strong Senate vote also appeared to be weakening resolve among House Democrats to block the measure. Senior Democrats said the House is likely to stage votes to amend the estate tax to assuage anger among the Democratic rank and file over what they viewed as an overly generous deal for the wealthy.

House Majority Leader Steny Hoyer, D-Md., told reporters that although "significant concerns" remain about the deal, the "overwhelming majority" of lawmakers in both parties think it's "absolutely essential" to approve a measure to prevent tax rates from rising next year.

House Democrats met into the evening outlining a strategy that focused increasingly on the estate tax. Outraged by the agreement to exempt individual estates worth as much as $5 million from taxation, senior House Democrats said they would press to lower the threshold to $3.5 million when the measure reaches the House floor this week.