Hours before the House passed its cap-and-trade bill, freshman Democrats Tom Perriello and Frank Kratovil were pondering the fallout of the votes they were about to cast in favor of a plan being denounced as "cap-and-tax."

"Maybe we should be called the conscience caucus," said Perriello, 34, who won his Virginia district last year by 727 votes even as Barack Obama was losing it by 7,512. He recalled Kratovil, 41, replying that perhaps they would be known as the caucus of soon-to-be unemployed.

Kratovil, who narrowly won a Maryland district that Obama lost to John McCain by 18 percentage points, acknowledged that "it would have been easier politically not to take that vote."

That some highly vulnerable Democrats in the House were willing to face tens of thousands of dollars worth of Republican attack ads as the price of supporting a bill to curb global warming is the untold story of what, so far, is the year's most dramatic legislative showdown.

Accounts of the battle typically focus on how many industry giveaways were added to the bill, how much it had to be weakened to round up the necessary votes, and how much pressure Speaker Nancy Pelosi and President Obama brought to win a 219-212 majority.

All true, but there was another dimension to the fight: A number of relatively young, politically vulnerable House members who had campaigned on promises to cleanse the environment decided that this vote was a risk worth taking. "A bill created by the old politics," said Perriello, "was passed by the new politics."

Well, yes, but old politics operated right to the end.

Take the cases of Kratovil and Colorado's Betsy Markey, another freshman Democrat from a district Obama lost. Both represent significant numbers of farmers. Their votes weren't secured until the chairman of the House Agriculture Committee, Rep. Collin Peterson (D., Minn.), had won last-minute changes protecting farm interests.

For Kratovil, one key was making sure that Maryland farmers, who already work under stringent state-imposed environmental standards, would receive the same benefits under the bill as farmers in states with less-demanding rules.

For Markey, major issues included helping rural electric cooperatives and protecting dairy farmers who feared the bill might impose a "cow tax" on cattle for producing methane by way of, shall we say, a thoroughly natural process.

Kratovil said he could not have supported a bill that hurt agriculture. With the amendments, he was free to vote in line with his core campaign promises: to promote "renewable fuel, to reduce U.S. dependency on foreign oil, and to protect the environment," including Chesapeake Bay.

Another factor is changing the political calculus - the rise of a substantial alternative-energy industry that encompasses wind and solar. For the first time, the political meaning of the word energy is not confined to oil and gas, even if old energy is still more connected politically.

Among the employers in Markey's district are Vestas Wind Systems A/S, a leading supplier of wind power, and Abound Solar Inc., a spin-off of research at Colorado State University that manufactures photovoltaic panels.

Markey said a large strip of her district was one of the most promising parts of the country for producing wind energy, and "this bill really helps our eastern plains."

Underscoring the dawn of a new energy politics were the eight Republican votes cast in favor of the bill, notably those of Mark Kirk of Illinois and Mike Castle of Delaware. Both are considering campaigns for the U.S. Senate next year, and they may see a future that others in their party don't.

Still, for potentially vulnerable Democrats who backed the bill, there will be short-run pain. Perriello and Markey were among 14 House members targeted by the National Republican Congressional Committee. In Perriello's case, a tough TV ad predicts huge increases in electricity prices.

Perriello said he was surprised that Republicans were "using information they know is fundamentally wrong." He planned to use the July Fourth weekend to speak in his district about the urgency of energy independence and the potential for renewable-energy jobs.

Perriello's fate will be a test of just how new our politics have become.

E-mail E.J. Dionne at ejdionne@washpost.com.