Calling all greens: Barack Obama, battling to remain the front-runner in the Democratic presidential primary, this weekend took on the most sacred cow in American politics: cheap gas.

Campaigning in Indiana, Obama distanced himself from the gas tax “holiday” proposed by Sen. John McCain, saying it may not bring down prices and would require raising other taxes to pay for highway maintenance.

"The only way we’re going to lower gas prices over the long term is if we start using less oil," Obama said in Anderson.

McCain pounced, saying through a campaign spokesman that "Americans need strong leadership that can deliver lower gas prices and a healthier economy, not Barack Obama’s inexperience and indecision." Obama’s Democratic rival, Sen. Hillary Clinton, did likewise, unveiling a new ad calling for suspension of the gasoline tax — a proposal first advanced by McCain on April 15.

As U.S. political campaigns go, the contrast between McCain-Clinton’s playing the gas-tax card and Obama’s brave clarity couldn’t be clearer.

What the 2nd Amendment is to the gun lobby, and lapel flag pins are to summer soldiers, energy prices that reflect and internalize costs are, or ought to be, to greens.

The dirty not-so-little secret about U.S. energy policy, the Original Sin underlying our 35-year-old energy crisis and the now-exploding climate crisis, is cheap energy, and Americans’ ingrained entitlement to it.

Low energy prices — kept low with tax breaks, ruined mountaintops, scarred lungs, and now-fracturing ice shelves — are what enabled suburbia, the “love affair” with the auto, and the hellspawn of SUVs that has begun engulfing China. Only high energy prices — prices that internalize the grievous costs of energy extraction and combustion via gas taxes and revenue-neutral carbon taxes — can instill the incentives and propagate the behaviors that will move us and other nations off of oil and off of carbon in the nick of time.

McCain has no clue. Clinton surely does, but can’t pass up a chance to pander. "Hillary Clinton knows it’s time to act, take some of the windfall profits of big oil to pay to suspend the gas tax this summer, investigate the oil giants for price gouging and collusion," her ad says. Right. Shadow-box at Big Oil while taxpayers and the climate absorb the punch. Here’s the ad:

An 18 cent-a-gallon 100-day “holiday” adding up to ten billion dollars may not be much in the grand budgetary scheme. It’s the principle that matters: closing the reality gap between the low price of energy and its outsize cost. With the Indiana and North Carolina primaries a mere ten days away, Obama’s stance shows that he not only gets it but is willing to stick his neck out for it.

Energy prices that tell the truth. This — not renewable fuel standards, not tough talk on Yucca Mountain, not even a moratorium on new coal plants — is this century’s defining energy policy issue.