By LISA LERER | 10/14/09 5:20 AM EDT

Climate-legislation supporters are increasingly turning to national security to bolster their pitch for a bill this year.

So far, the climate debate has largely focused on reducing greenhouse gas emissions that cause global warming, drafting an international climate change treaty and fostering new, cleaner sources of energy and so-called green jobs.

But for nearly two years, military and intelligence experts have been issuing studies warning that climate change could put American military personnel and national security at risk. Increasingly violent storms, pandemics, drought and large-scale refugee problems, they say, will destabilize regions and encourage terrorism. And American dependence on foreign energy sources will only exacerbate the threats and increase the likelihood of military action.

Now, with Massachusetts Democrat John Kerry emerging as a key player in the Senate climate debate, Democrats believe national security could emerge as a persuasive argument.

Kerry, chairman of the Senate Foreign Relations Committee, has been reaching out to Republican Sens. Dick Lugar of Indiana and John McCain of Arizona, who have long focused on U.S. security issues.

This week, Operation Free, a coalition of national security and veterans organizations, is sending a group of Afghanistan and Iraq war veterans on a 21-state, biodiesel-fueled bus tour to promote the message that climate change could hurt American security. The group was launched in August, a month after the House passed the climate and energy bill.

And Votevets, a left-leaning veterans group, bought $500,000 worth of radio ads featuring Iraq war veterans making the case that the climate bill would help the country become more energy independent and less reliant on oil from the Middle East.

“It’s not just a question of American energy; it’s a question of American power,” concludes the ad.

Some conservative Democrats who voted for the climate legislation in the House faced a backlash against the bill when they went home to their districts over the July 4 recess. Democratic leaders believe that a national security message could give their vulnerable members another line of defense to explain their vote in next year’s elections.

“If you talk about climate change in a way that discusses fragile states that are very vulnerable to its impacts, people realize that it’s our troops that will have to respond,” said John Powers, chief operating officer at the progressive Truman National Security Project, a member of Operation Free.

Climate change, say the organizers, threatens the security of U.S. borders and the country’s food and water supply. Failure to act, they say, could weaken America’s position in the world and the country’s credibility among allies.

In September, Operation Free organized a group of more than 150 veterans from across the country to visit Senate offices and the White House to raise awareness of the national security threats of climate change. They were joined by former Sen. John Warner (R-Va.), who had also served as Navy secretary and chairman of the Senate Armed Services Committee. Warner, along with Sen. Joe Lieberman (I-Conn.), sponsored a climate bill last year.

Kerry’s role as the sponsor of the Senate climate bill will also help spread the message that global warming is a security issue, say advocates, by virtue of his chairmanship of the Foreign Relations Committee.

At the unveiling of the climate legislation he sponsored with Sen. Barbara Boxer (D-Calif.), Kerry stressed the impact of the bill on national security.

“Fundamentally, this bill is about keeping Americans safe,” said Kerry. “Unless we act decisively, climate change could become a threat multiplier, a lit match on the kindling of an already dangerous world.

The intelligence community is also taking action on climate change

In September, the CIA announced it was opening a Center on Climate Change and National Security to examine how global warming could affect the country’s military strategies.