WASHINGTON - The Senate on Wednesday rejected a second effort by Republicans to deny union bargaining rights to 45,000 federal airport screeners.

However, in the face of a threatened veto by President Bush, senators decided to limit any union role to non-wage issues and give the government wide latitude to move workers around in an emergency.

On a 51-48 vote, the Senate adopted a measure by freshman Sen. Claire McCaskill, D-Mo., slightly weakening union protections put into the broad antiterrorism bill by Democrats on its Homeland Security Committee. The White House has threatened to veto the bill if it includes union rights for screeners and Republicans have boasted that Democrats don't have the votes in either the House or the Senate to override a veto.

The bill seeks to put in place some of the unfinished recommendations of the 9/11 commission report, including security upgrades on railroads, airliners, buses, trucks and pipelines. It also would provide money for state and local emergency communications systems and require more intelligence sharing among federal, state and local officials.

McCaskill's amendment gives the Transportation Security Administration the flexibility to reposition screeners and the power to "take whatever action necessary" during an emergency. "We are only trying to give to the screening officers in airports the same worker protections that we give so many of our men and women in uniform," she said.

Opponents said the amendment doesn't give the government enough flexibility.

"They have to plan like they're in an emergency all the time," said Sen. Tom Coburn, R-Okla.

Sen. Susan Collins, R-Maine, said the vote would set back the cause of screeners' employment rights because it meant the bill wouldn't be signed into law. Her amendment to give the screeners whistleblower but not union protections was on the losing side of a 53-46 vote. A similar effort Tuesday to strike the collective bargaining measure was defeated by 51 to 46.

"Everyone in this chamber knows that the president is going to veto this important bill if those provisions remain in there that the Senate just voted," Collins said.

The dispute over labor rights reflects a debate that stalled the creation of the Homeland Security Department in 2002. Sen. Max Cleland, D-Ga., voted against the bill because it didn't include collective bargaining rights; he subsequently lost his Senate seat to Republican Saxby Chambliss, who suggested Cleland was soft on terrorism.

Sen. Edward Kennedy, D-Mass., chastised lawmakers who said giving collective bargaining rights to screeners would make it harder to fight terrorists.

"What is it about the other side that questions that these are men and women that won't do their job when this nation is at threat?" Kennedy said.

Many screeners already belong to a union, the American Federation of Government Employees, but they don't have the right to bargain collectively.

Pay for many federal workers is set by statute and can't be negotiated. For some workers, such as those employed by the Federal Aviation Administration and the Security and Exchange Commission, the head of the agency sets their salaries and they can be bargained.

Though screener salaries are set by the head of the TSA, they were not given the right to bargain collectively when the agency became part of the Homeland Security Department.

The Senate amendment passed Wednesday would give the screeners collective bargaining rights, but specifically disallows bargaining for pay.

About a third of all of Homeland Security's 180,000 employees are unionized, including border patrol, immigration and customs agents.