WASHINGTON — The first round of budget hearings over the $100 billion the administration is requesting for the troop surge in Iraq will be the venue for the most serious challenge yet by Senate Democrats to the president’s Iraq war strategy.

Today’s hearing at the Senate Appropriations Committee pits the panel’s chairman, Senator Byrd, an 89-year-old Democrat from West Virginia, against Secretary of State Rice, Secretary of Defense Gates, and the chairman of the Joint Chiefs of Staff, General Peter Pace.

At stake is the money the president has said he needs to send five Army brigades to Baghdad and another brigade to Anbar province. Senate Republican leaders and the White House now fear that the negotiation process over the supplemental funding request for $100 billion will require the military to submit to conditions imposed by anti-war Democrats who will oversee the hearings for the budget request.

While Rep. John Murtha, a Democrat from Pennsylvania who leads the House caucus calling for a withdrawal from Iraq, has already promised the budget request hearings and subsequent votes will be the “real vote” on Iraq, other Democratic leaders have promised not to cut off funding for the troops.

But the promises by Senate Democratic leaders thus far have not included Senator Byrd, a former segregationist who has become his chamber’s leading proponent for restoring the power he says the executive branch has usurped from Congress to declare war. As one of the 23 Democrats who voted nay on the 2002 resolution authorizing force in Iraq, Mr. Byrd gave some of the most fiery speeches against the war, throwing down a pocket version of the Constitution and warning that his fellow lawmakers were abdicating the power to declare war that the Constitution guarantees Congress.

Mr. Byrd on February 1 introduced to little fanfare a binding resolution that would effectively expire the 2002 war resolution. “It will establish provisions to bring to a close U.S. military engagement in Iraq based upon conditions, not dates,” Mr. Byrd said upon introducing the legislation. “It will restore to Congress its constitutional war-making power by adding conditions that would terminate the original 2002 use of force resolution. The conditions can be summarized as follows: We have achieved our objective, we are no longer needed, or we are no longer wanted in Iraq.”

As chairman of the Senate Appropriations Committee, Mr. Byrd can use his leverage to hold up any supplemental request the administration makes for the troop surge. Yesterday, two Democratic staffers in the Senate told the New York Sun that they did not expect Mr. Byrd to threaten a cut off of funds. But nonetheless, they said that the supplemental funding the president is asking for will face heavy scrutiny. “We think there is an oversight role to be played here,” one of the staffers, who requested anonymity, said.

The White House at least is already anticipating the budget show down. President Bush yesterday appealed to the nation’s governors to help him defend his “strong budget” when it goes to Congress. “If you’re concerned about making sure your troops get what they need, make sure you call your congressman, or your senator,” he told the governors. The governors were also briefed by General Pace.

On the Iraq war funding specifically, Mr. Bush told the assembled state executives he was “looking forward to a healthy debate.” He added, “I’m also looking forward to defending, strongly defending the budgets, we send up to Congress, to make sure those troops who are in harm’s way have the resources and that we have the flexibility necessary to, and our commanders have the flexibility necessary to execute the plan we’ve laid out.”

This message yesterday echoed Secretary of State Rice’s views on the Sunday talk shows where she warned Congress against “micromanaging” the war.

A former senator and current Democratic governor of New Jersey, Jon Corzine, said yesterday he was troubled after Mr. Pace’s briefing. “The thing most disturbing to me — though most of the discussion was disturbing — was there was very little discussion on Afghanistan,” he was quoted as saying by the Newark Star Ledger.