The Senate took a huge step toward passage of the biggest increase in fuel economy standards for cars and trucks in three decades - but it came at a major cost for Democrats.

Backers of tougher legislation were forced to give up their plan to strip $21 billion in tax breaks, mostly to oil companies, and use the money to subsidize renewable energy lost by a single vote Thursday.

It was the second defeat for Democrats, after Senate Republicans, backed by the White House, blocked an effort to require utilities to get 15 percent of their power from renewable sources.

"Big oil and big coal succeeded in stripping out some very important parts of this bill," said Anna Aurilio, the Washington director of Environment America, the environmental arm of U.S. PIRG. "Nonetheless, it's still very important to get this fuel economy piece over the finish line.