The Lieberman-Warner Climate Security Act was a cap-and-trade bill, which would've capped carbon dioxide (CO2, a major contributor to global warming) from power plants, refineries and factories, aiming to cut emissions 71 percent by 2050. The economic costs of the bill were the major contention and won out over the cost of doing nothing for another year.
President Bush had promised to veto the bill. Both Barack Obama and John McCain sent letters to the Senate, advising that they would've voted for the bill. (Side note: Stephen Power at the Wall Street Journal has a good summary of each candidates approach and preferences on energy policy).
The National Resources Defense Council, for one, was still positive about the distance the climate bill went in the Senate. President Frances Beinecke told Time:
"We have taken comprehensive global warming legislation farther than it has ever gone before. A national limit on global warming pollution is inevitable."