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The details of a long-awaited US bill on climate change are to be made public later, but analysts are warning it faces a tough battle to be made law.
The bill, backed by Senators John Kerry and Joe Lieberman, will propose cutting US carbon emissions by 17% by 2020.
But it is also expected to propose easing restrictions on offshore oil drilling – likely to face opposition after the Gulf of Mexico oil spill.
Legislation on climate is a key part of US President Barack Obama’s agenda.
But the bill has been repeatedly delayed amid Republican opposition.
In March, Mr Obama announced restrictions on offshore drilling could be relaxed in a move analysts said was designed to win Republican support for the wider climate bill.
According to media reports, the proposals to be outlined by Senators Kerry and Lieberman will include protection for coastal states that do not want oil drilling off their shores.
Political constraints
The bill will propose setting a price on carbon emissions for large polluters such as coal-fired power plants, the Associated Press reported.
But the senators say the bill will exempt farms and most small and medium-sized businesses.
It will offer incentives of up to $2bn (£1.35bn) a year for firms to develop so-called clean coal technologies, including methods to capture and store carbon emissions.
And in another sweetener to the bill’s potential opponents, the legislation has several provisions aimed at boosting nuclear power.
But the bill is subject to a constrained political timetable.
Immigration laws have been moved to the top of the agenda and, with elections later in the year, it is uncertain whether the climate bill will even be discussed this year.
After the elections, the Democrats may well lose their stranglehold on Congress, making it much harder to get the bill passed into law.
"Everyone knows this is Congress’s last, best chance to pass comprehensive climate and energy legislation," said Mr Kerry.
He said a failure to pass the bill would mean Congress "will be rendered incapable of solving this issue".
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