Sens. Joseph I. Lieberman and John W. Warner are to introduce new climate legislation Thursday that will set tougher standards than a draft they unveiled in August.

The Connecticut Independent and Virginia Republican plan to talk about the measure in a discussion on the Senate floor.

The America’s Climate Security Act “will establish the core of a federal program to reduce U.S. greenhouse gas emissions substantially enough between 2007 and 2050 to avert catastrophic global warming,” according to a description of the bill.

“It will accomplish that purpose without harming America’s economy or imposing hardship on its citizens,” it says.

The bill would cover electric power, transportation and manufacturing sources that together account for 75 percent of U.S. greenhouse gas emissions.

Emissions caps would start at the 2005 level in 2012 and decrease annually, reaching 1990 levels in 2020 and 65 percent below 1990 levels in 2050.

The bill would retain the plan in the August proposal for free allocations of 20 percent of the emissions cap to manufacturing facilities in 2012 but would phase out the free allocation by 2036 instead of continuing it through 2050.

Oil companies would not receive any free allowances.

The bill would allocate 9 percent of the emissions cap free each year to states.

It would phase in up to 73 percent of the annual emissions cap for auction by 2036, compared to 52 percent in the August proposal. It would distribute 20 percent of auction proceeds to low- and moderate-income energy consumers.

The bill contains strengthened energy efficiency standards for residential boilers and for commercial and residential buildings.

Companies would be able to trade, save and borrow emissions allowances and generate credits when they get non-covered businesses, farms and others to reduce their emissions or capture and store greenhouse gases.

Earlier this month, six Democratic senators wrote to Lieberman and Warner urging them to strengthen their planned legislation.

The bill is expected to be the primary Senate vehicle dealing with climate change. Sen. Barbara Boxer, D-Calif., chairwoman of the Senate Environment and Public Works Committee, plans to mark up the legislation this fall.

Lieberman and Warner are the chairman and ranking minority member of the panel’s Subcommittee on Private Sector and Consumer Solutions to Global Warming and Wildlife Protection.