A coalition of labor unions - traditionally a bastion of support for Democrats - are lobbying senators of that party in favor of President Bush's air-pollution plan.

The unions have written to members of the Senate Environment and Public Works Committee, where they are targeting freshman Sen. Barack Obama (search), D-Ill., to possibly provide the swing vote when the measure is considered Wednesday.

Tommy Vietor, a spokesman from Obama's office, said Monday that the senator welcomes the unions' opinion on the bill, but he couldn't say whether it would affect Obama's vote.

Support for the measure is divided down the middle of the 18-member committee. Democrats on the committee, along with Sen. Jim Jeffords, a Vermont independent, and moderate Republican Sen. Lincoln Chafee of Rhode Island, oppose the measure because it doesn't deal with carbon dioxide (search), the chief "greenhouse" gas blamed for global warming (search).

The Bush plan would allow states and utilities to have a pollution trading system in which plants unable to meet the required reductions could buy emission allowances (search) from other plants that have exceeded the required cuts.

It includes three other pollutants: mercury (search), a toxic metal; sulfur dioxide (search), which forms acid rain; and nitrogen oxides (search), a contributor to smog.

Republican senators support the Bush proposal and hope opponents will at least pass it out of committee so it can be debated on the Senate floor. The Bush administration has been trying for three years to get Congress to endorse it.

In a letter sent last week to Jeffords and committee Chairman James Inhofe, R-Okla., the unions said the bill is "timely and necessary" and contains emission reductions that they support.

"Members' jobs and incomes are sensitive to the cost and reliability of energy in all sectors of the economy, including manufacturing," the letter said.

The coalition of unions includes the United Mine Workers (search), Teamsters (search), Boilermakers (search), Electrical Workers (search), Utility Workers (search) and the Transportation/Communications International Union (search).

Bill Cunningham, president of Unions for Jobs and the Environment (search), the Washington-based group that was spearheading labor support for the measure, said unions are hoping their influence with Democrats will help pass the bill, which he says will protect jobs.

"The vote is very close, so we wanted to ensure the senators that there is union support," Cunningham said.