The contest for the Democratic presidential nomination heats up again Tuesday with the Pennsylvania primary, which kicks off the last round of primaries and could loom large in the battle between Sens. Hillary Clinton and Barack Obama.

The primary also happens to fall on Earth Day, a fitting date in a race where all three candidates have made the environment part of their platforms.

All three support a carbon cap-and-trade program, for example. Clinton and Obama call for a system that would auction off 100 percent of emissions permits, which they claim would cut carbon-dioxide emissions 80 percent below 1990 levels by 2050. Sen. John McCain’s plan would reduce emissions 65 percent by 2050, according to USA Today.

The plans have led some, such as Environmental Defense president Fred Krupp, to call such a program “inevitable.” Others, such as Dan Reicher, director of climate and energy initiatives for, have said it still could take longer to create such a program than voters expect (see McCain, Obama, Clinton Reps Talk Energy and When Will the Climate Change for Climate-Change Legislation?).

The candidates differ in their stances on nuclear power. McCain supports the use of nuclear power, while Clinton doesn’t see it as “the first option,” but also isn’t ready to shut down nuclear plants already in operation, according to Gene Sperling, chief economic advisor for the Clinton campaign.

Obama reportedly has said that nuclear power isn’t “necessarily our best option,” but also that it should be explored as part of the energy mix, according to CNN and the Dallas Morning News. At last month’s ECO:nomics conference in California, Jason Grumet, principal energy advisor for the Obama campaign, said the senator sees problems with “current” nuclear technology (also see this Environmental Capital post).

All three candidates have called the environment an important issue, yet questions about global warming in televised debates and interviews are few and far between, according to the League of Conservation Voters. Out of 3,231 questions by political reporters from five networks, only eight were about global warming, according to a league tally (via Newsweek).

According to the latest league scorecard, Clinton has an 87 percent "lifetime" rating, while Obama has an 86 percent rating and McCain has a 24 percent rating. The scores rate the candidates’ congressional voting records on environmental legislation throughout their careers.

You can read more about the candidates’ environmental platforms on their sites (click here for Clinton, here for McCain and here for Obama), and in our Greentech Voter Guide (which also includes cleantech campaign contributors and the candidates’ cars) here.

We canvassed the greentech community, calling more than 50 industry insiders to ask for their thoughts on the primaries and on the candidates’ environmental platforms. Most declined to get behind a candidate. Here are the answers from a brave few who agreed to discuss who they would like to see win and why:

Joel Makower, chairman of Greener World Media, which produces
“I'm supporting Obama, for a lot of reasons, not just environmental. But I believe he gets the opportunity more than the others. In the most recent debate in Pennsylvania, he mentioned Apollo Alliance — one of the few times that the organization's name has been uttered in this campaign. The mere fact that he clearly sees the connection between clean energy and economic development is nothing short of revolutionary, as far as presidential candidates go.”

Peter Liu, founder and vice chairman of New Resource Bank:
"I voted for Obama in the California primary, not solely based on environmental issues but [because of] the breadth of his political message, and am inclined to vote for him in the general election if he's the party's candidate. I … have a lot of respect for many of the people advising him on environmental issues. If we take appropriate comprehensive action [on climate change], it will take us out of the current economic downturn and help reinvent and jumpstart our economy. … Even if it's conservative leadership, I think it will be an improvement over current politics. I've met McCain personally and believe his leadership is true. Bottom line, I think whoever wins will be a welcome change to what we have now and will be a progressive step forward from an environmental standpoint."

John Giddings, author of The Solar Evangelist blog:
The thing to keep in mind about getting cleantech-positive legislation through is that it’s not going to be about a politician's ideology. Politicians will respond to their constituency. Right now, the voting public is uniformly in favor of stimulating cleantech industries such as solar, so the question is not about which candidate is more clean-positive. It’s about who is the more accomplished politician. For now, that is Hillary. In November, both candidates will be making promises to promote cleantech and again the question will not be who is "most green" but who can work the machine to pass legislation.

Stephan Dolezalek, a managing director at VantagePoint Venture Partners:
“I haven't decided on whom to support because, so far at least, I don't think any of the three remaining candidates has shown a real understanding of the issues involved. As a result, I find their proposals to be ‘politically correct’ but largely uninspired and insufficient to get us where we need to go.”

Rob Wilder, CEO of WilderShares, which manages clean-energy indices:
“I ought to stay out of this one. But I will say that compared to how things looked 10 or 12 months ago, we ended up with a field [of candidates] who are very pro-clean energy. It’s a good field and the remarkable thing is that the leading Republican candidate is so pro-clean energy; it’s a dramatic turnaround. George Bush now also is very different from the Bush of eight years ago. Still, any of the three would certainly be more conducive to clean energy and to taking science more seriously.”

Mark Stowers, vice president of research and development for Poet, an ethanol company based in South Dakota:
"I'm not decided yet. We are still waiting to see what happens on the democratic side, to see where candidates' positions finally settle. We're just waiting to see the nominees and then the campaign will begin. In South Dakota, for example, we have a very late primary, so it's unlikely we are going to have very much of a decision. … How the candidates look at ethanol in particular will be an important factor at how I look at the election. Their continued support of the Energy Independence and Security Act will be very important. Their support of furthering the increase in ethanol blends in transportation fuels, blends like mid-level blends and E85, for examples, would be another area of interest."

Adam Browning, executive director of the Vote Solar Initiative:
"I’ve been on the O-train for a long time. The reasons range from pragmatism to optimism. … It’s pretty simple. I just like [Obama] better — both his personality and politics. And he has three things going for him. He has good instincts. He can get elected. And once elected, he can get things done. I trust him.”