The nonpartisan congressional Joint Committee on Taxation boosted the cost estimate of President Bush's proposed tax cut yesterday by $31 billion, to $726 billion through 2013, with the plan to slash taxes on dividends taking $396 billion of that total.

The new estimate is important because it will serve as the plan's official price tag as Congress tries to establish a blueprint for spending and taxation for the next fiscal year. The Treasury Department had put the cost of the plan at $695 billion over the same time frame.

The estimate surfaced as Treasury Secretary John W. Snow began the administration's formal defense of the tax cut during the first of four hearings scheduled by the House Ways and Means Committee. Also yesterday, seven Senate moderates from both parties began meeting to try to find consensus on a smaller tax cut.

Snow acknowledged that the Bush dividend proposal was facing tough questions on Capitol Hill. Speaking before business lobbyists at the U.S. Chamber of Commerce, he said opponents of the plan who were expressing concern over the prospect of record budget deficits were merely making a "convenient" argument, "not a substantive attack."

Hours later, Ways and Means Committee Democrats hammered Snow and the administration for what they called an unprecedented push for deep tax cuts as the nation prepares for war. "How in the world can you sit there with a straight face and sell that to the American people?" asked Rep. Jim McDermott (D-Wash.), a vocal opponent of war with Iraq. In hectoring tones, the Democrats implored Snow to offer a cost estimate for the war and the planned occupation of Iraq before Congress is asked to vote on the tax cut.

Snow tried to keep the economic proposal and the war on separate tracks. "We can afford the war, and we'll put it behind us," he told lawmakers, "but we have to be sure the economy is growing."

But some Republicans also raised war costs as a tax-related issue. Sen. John McCain (Ariz.) said he needed to get a firmer grasp on war costs and the price tag of a prescription drug plan for seniors before he can decide on the size of a tax cut. "I wonder if we should be debating [tax cuts] in the middle of a war, or at the start of a war," he said. Sen. Susan Collins (R-Maine) echoed his comments.

Staff writer Helen Dewar contributed to this report.