WASHINGTON - The White House threatened yesterday to veto a Senate bill to create a vast new Homeland Security Department, signaling its sharpest objections yet to a measure crafted by Democrats who say they want to make sure the agency protects its workers' rights.

The threat came after Republicans on a Senate committee failed to revise the bill to give the president more flexibility in hiring, firing and other personnel matters at the department, which is to include 170,000 workers.

President Bush's spokesman, Ari Fleischer, said senior White House advisers would recommend a veto if a bill that closely resembles the Senate version reaches Bush's desk.

"The president will continue to work in a bipartisan manner," Fleischer said, "but we do have serious reservations about the direction the bill has taken."

As the president began a speech on health care in High Point, N.C., he said: "I just want to make sure that Congress understands that when we do create this department, I've got to have the ability to manage the department in a way to make the homeland more secure."

The Republican-led House began considering its version of the legislation last night, after intense negotiations among party leaders over what changes Democrats would be allowed to propose.

Both bills would fulfill Bush's request to transfer numerous agencies to the new department, including the Coast Guard, Secret Service, Border Patrol, Customs Service, Federal Emergency Management Agency and the new Transportation Security Administration.

But the Senate Governmental Affairs Committee voted 12-5 yesterday to approve a version of the bill - drafted by Sen. Joseph I. Lieberman, the Connecticut Democrat who chairs the panel - that includes several items the White House vehemently opposes.

The Senate is expected to consider the measure next week. Despite the split over the rights of civil service workers, the differences are expected to be resolved, and Bush is expected to sign some compromise.

Still, to send a message, White House officials went on the offensive yesterday. They said the Senate bill would strip the president and a new secretary of homeland security of the authority and flexibility they need to run an agency whose mission is to safeguard the nation.

Senate Democrats bristled at the veto threat, with Lieberman saying that he was "dismayed" to hear of it.

Ranit Schmelzer, a spokeswoman for Senate Majority Leader Tom Daschle, a South Dakota Democrat, said it was "highly premature" for the White House to threaten a veto before the legislation had emerged from committee, gone to the full Senate and been sent to a conference to reconcile differences between the House and Senate approaches.

"I don't think we're at the point where we want to be drawing lines in the sand, but the civil service issues are important," Schmelzer said. "If the bill gets broad support and the president threatens to veto it, he will be seen as standing in the way of his own department."

Bush administration aides and congressional Republicans said it was vital that Bush take an assertive stance on the measure early, because of the speed with which Congress is acting on it.

"A message needed to be sent now, to send a signal to senators that the president believes aspects of the bill would not make a good department," an administration official said, speaking on condition of anonymity. "You have got to lay a marker down."

Senate Republicans said Bush needed to show that he would not hesitate to challenge Democrats on the labor issues, and will do whatever it takes to build an effective Homeland Security Department.

Sen. Fred Thompson of Tennessee, the senior Republican on the Governmental Affairs panel, said Bush is "just letting it be known that his desire for a homeland security bill is not so great that he's willing to let his people take on this huge new responsibility with one hand tied behind their backs."

Much of the dispute focuses on a provision that would protect the collective bargaining rights of federal employees transferred to the new department. It closely parallels a proposal that Rep. Constance A. Morella, a Montgomery County Republican, is trying to add to the House measure. Morella's amendment would allow workers to belong to a union unless they are assigned new duties that relate specifically to the war on terrorism.

Such protection is needed, Morella and many Democrats argue, because current law gives the president the power to waive collective bargaining rights for employees whose jobs involve intelligence-gathering or national security.

Representatives of organized labor have been lobbying to keep the language in the Senate bill and add it to the House version. Without it, union lobbyists say, workers have no guarantee that their rights will be protected.

"Basically, what they're saying is, 'Well, trust us,'" Chuck Loveless, director of legislation at the American Federation of State, County and Municipal Employees, said of the Bush administration. Though the union's members would not be directly affected by Bush's plan, its leaders have joined other labor lobbyists in opposing it as a dangerous precedent.

"There is just a complete level of distrust in terms of where they would go with this," Loveless said.

But a Bush administration official insisted that the president's demands are not designed to ride roughshod over worker rights. "This is not union-busting," the official said.

"You need to know that there is a lot of really fine federal government employees working a lot of hours to protect you," said Bush at fund-raising dinner in North Carolina last night. "We can just do it a better way."

Democrats, however, said the homeland security debate is not the place for Bush to make a point on union issues.

"It's like they're trying to take these same old right-wing attacks on labor rights and bring them into this," said Sen. Barbara A. Mikulski, a Maryland Democrat. "If we're going to protect the homeland, we should also look out for the very federal workers who are going to be protecting their homeland."

Bush has concerns, officials said, that Democrats are not granting the administration enough power to shift funding from one part of the new department to another without congressional approval.

And he opposes a provision in the Senate bill that would require congressional confirmation of the top White House adviser on terrorism.

"That's a nonstarter," Fleischer said. "That's micromanagement of the White House by the Congress."