While the fate of both bills is uncertain, there is little doubt a threshold has been crossed.  Automakers, bowing to a changed political and economic landscape, are now willing to support fuel efficiency standards that for years they said would be impossible to meet. After six years of near silence on the issues, President Bush this year has endorsed stringent new mileage rules, a sharp increase in production of renewable fuels and concerted international action on climate change.  For years, efforts in Congress to impose strict fuel economy rules were stymied by a coalition of pro-business Republicans and auto-state Democrats unwilling to dictate to the Big Three carmakers. More recently, broad legislation to reduce greenhouse gas emissions has been stalled by opposition from electric utilities, coal producers, oil refiners and factory owners afraid of the costs of compliance.  “All that has changed,” said Representative Edward J. Markey, Democrat of Massachusetts, who has been a frustrated advocate for automobile fuel efficiency and energy conservation legislation in his 15 terms in Congress. “Things are now dramatically and in a telescoped time frame all coming together to address these issues that have been on a 30-year detour.”  On a less cosmic scale, a change in the domestic political climate has been critical, too. When Democrats took control of the House and the Senate in January, they declared that energy legislation would be a top priority and set this month as an informal deadline for action.  Representative John D. Dingell, the Michigan Democrat who is the senior member of the House and a longtime advocate for the auto industry, concluded this year that some form of fuel economy bill was coming. He warned top auto executives that he would do his best to protect the industry from unworkable measures but that they should begin to retool for a new, fuel-constrained era.  Mr. Dingell waged a valiant fight with Speaker Nancy Pelosi on behalf of the auto industry last week and won important concessions. But the 35-m.p.g.-by-2020 standard stayed in the bill and is expected to become law.  “For almost 20 years, automakers and environmentalists have been locked into an often-vitriolic and rarely productive fight over fuel economy,” said Jason Grumet, executive director of the bipartisan National Commission on Energy Policy. “What has happened in the last couple of years is that you have had a number of additional and very potent voices join the discussion. The upheaval in the Middle East has crystallized recognition that these issues are much bigger than just how many jobs in the upper Midwest are affected. Other industry leaders, outside of the auto industry, are starting to express concern that the volatility of oil and gasoline prices are exposing a fundamental weakness in our economic competitiveness.”  House leaders on Tuesday evening were still writing the energy legislation that will reach the House floor this week. In addition to incentives for a huge increase in production of biologically based fuels like ethanol, the package is expected to include a mandate that electric utilities produce 15 percent of their power from renewable sources by 2020. House leaders also plan to include $21 billion in new taxes on the oil industry and other interests to pay for the package.  Some aides expressed doubt that the Senate could pass the renewable power mandate or the tax bill, both of which Mr. Bush has threatened to veto.  Business interests are beginning a multipronged campaign to defeat the House energy bill and the Senate climate change legislation, claiming both will impose unacceptable costs on American industry. Electric utilities, manufacturers and the United States Chamber of Commerce began a series of print and television advertisements against the measures late last week.  Bill Kovacs, the chamber’s vice president for environment and regulatory affairs, said the bills would do little to increase domestic production of energy supplies and would make it hard for industries to compete against foreign companies with fewer regulatory or environmental shackles.  Mr. Kovacs said that opposition to these measures cut across partisan and regional lines, depending on where power lines or wind farms are located, for example, or whether increased ethanol production results in higher food or fuel costs.  But advocates for the bills can and do make the same argument, saying that support for increased fuel economy and measures to combat global warming are now bipartisan and nationwide.  Senator John W. Warner, Republican of Virginia, is a sponsor of the global warming bill. Mr. Warner opposed weaker legislation when it was proposed two years ago, but said recently that he had changed his mind because of two factors: “science and my grandchildren.”  A version of this news analysis appears in print on , on Page A22 of the New York edition with the headline: Crossing a Threshold On Energy Legislation. Order Reprints| Today's Paper|Subscribe
Congress is poised to act on the first bill to increase vehicle fuel efficiency significantly since 1975 and on the first economywide bill to address global warming since scientists raised the alarm in the late 1980s.