Our attention spans — and response times — just got shorter.

Thanks to President Barack Obama, Ashton Kutcher, Oprah Winfrey and the Iranian election contretemps, the micro-blogging social media platform Twitter has gone mainstream.

The evidence is still anecdotal, but it strongly suggests that Twitter has become yet another weapon in the public policy advocate’s arsenal.

One of the best examples came this summer, when the House began considering the American Clean Energy and Security Act, which includes a cap-and-trade carbon dioxide program designed to reduce economy-wide greenhouse gas emissions 17 percent by 2020.

As the House was poised to pass the 1,200-page-long bill, some of the fiercest debates were confined to 140 characters. Twitter, of course.

One of the biggest squabbles about ACES, both online and offline, has been how much it will cost consumers. House Republicans have said the bill would raise annual household energy costs by more than $3,000 per year, while the Congressional Budget Office and the Environmental Protection Agency have each conducted studies putting the number in the $100-to-$200 range.

With that debate in mind, former Vice President Al Gore — No. 21 among the Twitterati with more than 1.15 million followers — used Twitter to arm activists with a messaging point: the climate change legislation would reduce carbon dioxide emissions while costing just “a postage stamp a day.—

Sure enough, the postage stamp line became a rallying cry among environmentalists, and it was mentioned in the Associated Press and other news outlets the next day.

Just before the House voted on the bill, Gore issued a call to action — via a tweet — encouraging people to join his “urgent conference call,— in which the Nobel Prize winner discussed the legislation and how activists could help speed its passage.

When it comes to Capitol Hill policy debates, why use sites like Twitter and Facebook to sway public support? Much like everything else, it comes down to money. And attention … and that currency is golden here.

With advocacy budgets dwindling for TV and radio, participating in social media doesn’t cost anything (besides staff time) to reach an interested, and potentially influential, audience.

Using hashtags (keywords with a “#— in front, such as #climatebill, #Waxman-Markey and #ACES), Twitter users can target the very people who are plugged into the climate change debate. With the climate change action picking up in the Senate, the hashtags are climbing Twitter’s “Trending Topics— list of popular discussions.

The hope is to sway the influential participants who have thousands of followers on Twitter. By being timely and provocative, a good tweet can entice these influential people to “retweet— that message, amplifying it and helping it spread virally.

Active Twitter participants in the climate change debate include environmental groups like the Sierra Club (@SierraClubLive), the National Resources Defense Council (@NRDCLive); reporters and bloggers like Andy Revkin (@revkin) of the New York Times, and industry groups like the Alliance of Automobile Manufacturers (@auto_alliance) and the American Coalition for Clean Coal Electricity (@AmericasPower).

And just as Americans choose their cable news channel based on their political preferences, Twitterers organize themselves by hashtags.

Those against the Waxman-Markey bill tend to use the hashtag #capandtax or #tcot (which stands for “top conservatives on Twitter—), while environmentalists have adopted the topics #green, #nocoal and #powershift09.

Members of Congress themselves have gotten into the act. About one-quarter of Members in both chambers are among the Twitterati.

Sen. Claire McCaskill (D-Mo.) has taken to Twitter with such fervor that U.S. News & World Report has called her “Senator Tweet.— She sends out a few tweets per day, including this one from July 9: “Twitter is great. So many can weigh in. Quickly and easily. Wish more of my colleagues used this short cut to get people’s views on issues.—

In April, McCaskill wrote on her Tumblr blog that she uses Twitter to post information about serious public policy issues. “Short and sweet,— she wrote, “these messages are intended to drive thought and discussion rather than provide a thorough analysis of the issue.—

Since Members of Congress use Twitter, the medium has become a must-read platform for people trying to influence public policy debates.

MoveOn.org pounced on McCaskill when she tweeted that she had concerns that the climate change bill would end up hurting the economy. MoveOn sent e-mails to its massive member list, attacking McCaskill for “repeating Republican talking points about clean energy.—

The group asked Missouri voters to call the Senator to complain.

But that wasn’t all. MoveOn also asked its members to voice their unhappiness with McCaskill on Twitter, providing a sample tweet referring to the Senator by her Twitter account, @clairecmc.

Unlike Facebook, Twitter still has a young user base with the age group 20 to 24 as the biggest demographic, according to the social media monitoring service Sysomos.

Young people, of course, were highly visible during the 2008 presidential election, but they often fade away when it’s time to get into the nitty-gritty of governing. As a result, Twitter has become the vessel of choice for communicating with a demographic that doesn’t traditionally make its voice heard during policy debates.

Despite Twitter’s (current) youthful constituency, older demographics are adopting social media sites at the fastest rate. And social media have become online experiences that transcend demographics and dominate Internet usage for nearly every age group.

These interactive online technologies that allow online users to share content have made themselves indispensable, and the features offered by social media sites are firmly entrenched as the primary method for online dialogue.

Anyone who wants to shape a public policy debate needs to master social media … in 140 characters or less.

David Haase leads the editorial team @Virilion, the D.C.-based interactive agency. In pre-social media days, he wrote a syndicated column, “Plugged-in Politics.— (Exactly 140 characters)