Allies of the U.S. auto industry stepped up a campaign yesterday to soften strict vehicle fuel-efficiency mandates in proposed energy legislation before the Senate, even as momentum for the tougher measures continued to build. Auto lobbyists said they were encountering stiff resistance on Capitol Hill. They said they felt like the industry was being punished for what one called the sins of the past" -- successfully beating back attempts to make major changes to the nation's vehicle mileage laws. Yesterday, Sen. Dianne Feinstein (D-Calif.) defended the current bill, arguing that it would provide flexibility for automakers. "There are all kinds of dire warnings," Feinstein said. "The fact of matter is that Detroit has done nothing about mileage efficiency for the past 20 years, and the time has come."