For the first time, the Bush administration is endorsing mandatory requirements for heightened security at chemical plants, many of which homeland defense experts consider highly vulnerable to catastrophic terrorist attack.

The change in policy is one of the first enunciated by new Homeland Security Secretary Michael Chertoff, who is conducting a top-to-bottom review of the two-year-old department's priorities and organizational chart.

Until this week, administration officials had embraced the chemical industry's proposals for voluntary security precautions, though they had warned that the day might arrive when industry foot-dragging would compel a crackdown.

The new Bush administration stance is outlined in testimony to be delivered today by Robert Stephan, recently named the Homeland Security Department's undersecretary for intelligence and infrastructure, at a Senate hearing. A transcript was made available by Senate staff members.

"I can report on his behalf that Secretary Chertoff has concluded that . . . the existing patchwork of authorities does not permit us to regulate the industry effectively," Stephan is to tell the Senate Homeland Security and Governmental Affairs Committee. "While most companies have been eager to cooperate with the department, it has become clear the entirely voluntary efforts of these companies alone will not sufficiently address security for the entire sector."

U.S. officials say that an attack on some chemical plants in and near large cities, including a number in northern New Jersey, could cause hundreds of thousands of deaths if a resulting chemical cloud were spread by wind. Attacks on any of scores of other sites could result in thousands or tens of thousands of casualties, they said.

"I'm pleased the administration is endorsing the need for legislation dealing with the chemical sector," said Sen. Susan Collins (R-Maine), the committee's chairman. "In the past, the administration's position has been ambiguous."

Administration officials have praised the work of some large chemical companies, such as Dow and DuPont, and their trade association, the American Chemistry Council, for developing a voluntary security regimen that involves actions such as cataloguing on-site chemicals, upgrading training of security teams and installing more sophisticated surveillance equipment.

But many smaller chemical companies -- some of which operate at times with minuscule profits and are highly secretive about their operations -- have resisted voluntary security arrangements.

"Anecdotal information of poor or nonexistent security in this sector is overwhelming," Richard Falkenrath, formerly President Bush's deputy homeland security adviser, testified before Collins's panel in April. "There has been no significant reduction in the inherent vulnerability of the most dangerous . . . chemical facilities" since Sept. 11, 2001.

Falkenrath, now a Brookings Institution fellow, echoed the view of many members of Congress who say Homeland Security has moved too slowly to tighten chemical plant security. He said he noted "two disturbing tendencies" among many U.S. officials handling the issue: They try to avoid government security requirements at all costs and are passive about surmounting minor bureaucratic obstacles on the issue.

In his advance testimony, Stephan does not detail the type of regulation sought by DHS, except to say the most dangerous sites would receive the most government scrutiny, and companies would be allowed to choose from among menus of security options.

Collins said she and the panel's top Democrat, Sen. Joseph I. Lieberman (Conn.), plan to craft a bill mandating security steps for chemical plants. For more than a year, Democrats and Republicans on another Senate committee, Environment and Public Works, fought to a stalemate over competing chemical security bills.

The chemical council has dropped its opposition to mandatory security in part because several states are drawing up their own chemical security laws, creating the danger of a hodgepodge of regulations, officials said.

U.S. officials say that an attack on some chemical plants in and near large cities may cause hundreds of thousands of deaths. The plant pictured, in Port Elizabeth, N.J., is along the "most dangerous two miles in America."