Seen from outside a political lens, it should have been an easy choice for Clinton: Tires are so toxic that even limited exposure can cause permanent health damage, especially to children, whose developing brains and immune systems are hypersensitive to the pollutants tires produce. Tire pollutants can cut years off a child's life and impair mental development, according to the American Lung Association. For someone who had once been chairwoman of the Children's Defense Fund and who had forged her political identity around protecting kids' well-being, it would seem like a no-brainer.

But in 2005, Clinton had things on her mind in addition to children: She was determined to boost her margin of victory in upstate New York in her 2006 Senate reelection campaign -- even if it meant sacrificing children's health for, at most, a few hundred votes from people who bought into IP's empty threats.

And so, despite the pollution concerns, Clinton went along with IP and lobbied to allow it to go ahead with a two-week test tire burn. Although that may not sound like a lot of time, tires are so toxic that the acrid cloud they produce can cause damage after even just a few hours of exposure.

The tires turned out to be so polluting that the emissions exceeded even IP's extremely lax permit. The company was forced to suspend the incineration three days after it started. It didn't go out of business, but Clinton had provided her critics with more evidence that political calculation was her real first priority.

Obama hasn't been any better in terms of acting on principle:

Exhibit A is Obama's enthusiastic support for "coal to liquid" technology, which allows auto fuel to be squeezed out of coal. Obama touts it as a way to free America from reliance on Saudi oil fields and to tackle global warming. However, coal-to-liquid technology produces twice the amount of greenhouse gases that regular old oil does; additionally, it's so expensive that it's unlikely to displace one drop of cheap Saudi oil anytime soon.

So why would he support it? What's more, why did he vote for other anti-environment policies, such as President Bush's 2005 energy bill, which funnels more than $27 billion in taxpayer subsidies to big polluters?

A huge factor in Obama's decisions was his desire to support Illinois agribusiness (Bush's energy bill contained massive ethanol subsidies) and the southern Illinois coal industry. His votes mean that he's willing -- sometimes, at least -- to put these kinds of parochial interests ahead of the global environment and Americans' health (pollution from coal-fired power plants kills more than 30,000 people every year, according to EPA consultants Abt Associates).

Obama has explained his positions by saying that sometimes you need to "trim your sails" -- by which he means cutting back on goals to avoid becoming marginalized.

But it's exactly that kind of political calculation -- special interests versus doing what's right -- that Obama is promising to reject. Obama's energy policy shows that so far he is at least as much a creature of establishment influence-peddling as Clinton is.

Fortunately, John Edwards and New Mexico Governor Bill Richardson provide some hope:

While both Democratic presidential candidates have in the past surrendered to big polluters on key issues, lately they've been showing real grit when it comes to defending the planet.

Edwards has called for a ban on construction of coal-fired power plants that don't capture all their greenhouse gases. He also has released an ambitious plan to cut global warming pollution by 80 percent by 2050. Richardson has one-upped Edwards by proposing the same cuts by 2040 and speedier conversion to clean electricity sources and dramatic cuts in oil consumption.

Edwards and Richardson seem to have learned that at least when it comes to energy policy, courage can trump calculation.

Thanks to David Roberts at Grist in his really nice blog post about this article for pointing out that Chris Dodd also has a great climate plan.

Glenn Hurowitz, who blogs at DemocraticCourage.com, is the author of the forthcoming book Fear and Courage in the Democratic Party.