President Obama pauses while speaking during an online Twitter town hall meeting from the East Room of the White House July 6, 2011. (Brendan Smialowski/GETTY IMAGES)

The bully pulpit has a new kind of altar call: “Tweet them. We’ve got a hashtag. Here’s the hashtag for you to tweet them: #dontdoublemyrate.”

President Obama repeated that Twitter hashtag twice more during a Tuesday speech opposing an increase in student loan interest rates. For good measure, he even had his Chapel Hill, N.C., audience chant it back to him.

Within moments after Obama finished his remarks, Twitter users had written more than 20,000 posts containing “#dontdoublemyrate” — enough for Twitter to declare it a top 10 worldwide “trending topic.”

The six-year-old microblogging site came into its own this presidential cycle, but the past few weeks have demonstrated how clearly it has become the tool of choice for getting something into the political bloodstream, from manufacturing a battle over who can be called a working mom to building a movement around a piece of legislation.

And the fact that the president is now incorporating hashtags into his speeches shows how Twitter is redefining the means by which politicians shape, distribute and refine their messages. Campaigning in 140 characters or less provides almost instant feedback, which campaigns use to figure out what is working and what isn’t, even before it hits the blogs, much less the traditional media outlets.

But as both sides are learning, once they enter the Twitterverse, the digital currents can sweep in many directions.

Conservatives seized upon Obama’s #dontdoublemyrate hashtag to complain about gas prices and unemployment. And the GOP-leaning U.S. Chamber of Commerce quickly bought space on Twitter to assure that everyone searching for the hashtag saw its tweets at the top of their computer screens.

Within 45 minutes of Obama’s speech, Speaker John A. Boehner (R-Ohio) had picked up the hashtag on his Twitter feed — to argue that Democrats, not Republicans, were responsible for the fact that student loan rates are set to double.

“Twitter has become the ultimate real-time engagement tool for our campaign,” said Zac Moffatt, digital director for GOP presidential hopeful Mitt Romney, whose operation has grown noticeably more aggressive in its use of the social media service.

In the past, for instance, the campaign might have sent out a mass e-mail to protest if a Democratic cable television pundit accused the candidate’s wife of never working a day in her life. And quite likely, it would have sunk without a ripple.

But when that happened at 8:43 p.m. on April 11, Ann Romney issued her first tweet: “I made a choice to stay home and raise five boys. Believe me, it was hard work.” And it caused a sensation that threw the Democrats on the defensive on women’s issues.

Although almost no one on Twitter had been mentioning Ann Romney or strategist Hilary Rosen’s name when Rosen made her comment on CNN, the number quickly jumped to more than 150 tweets per minute. And by noon the next day, more than 2,800 Twitter users had sent Ann Romney’s tweet on to their tens of thousands of followers.

Twitter is dwarfed in size by Facebook, with 24 million active U.S. users at the end of last year, compared with nearly 133 million for Facebook, according to an estimate by the research firm eMarketer. But it is growing more than twice as fast, at 31.9 percent last year.

What gives Twitter its outsize influence in politics is not how many users it has but who those users are — journalists, elected officials, campaign operatives.

About 90 percent of senators and House members now have Twitter accounts, which is triple the percentage at the beginning of last year, said Adam Sharp, a former congressional aide and C-SPAN producer whom Twitter hired in 2010 to be its emissary to the political world. So do 42 governors and more than 35 world leaders.

“Every single thing we do, there’s a tweet button next to it,” said Katie Hogan, a spokeswoman for Obama’s campaign. “We can now reach not only our supporters, but our supporters can drive the message, too.”

It also has an immediacy that not even Facebook can achieve. “It has replaced the 6 p.m. deadline,” said Moffatt, liberating the campaigns from the traditional nightly news cycle.

Sometimes, the way messages spread can surprise even those who spread them.

For instance, when The Washington Post reported on April 5 that Romney was taking advantage of a rule exception that allowed him to avoid disclosing the nature and extent of his financial holdings, the Obama campaign quickly came up with the hashtag #whatsromneyhiding. It almost instantly shot to the top of the worldwide “trending topics” list, Hogan said.

The Romney campaign’s Moffatt says that a few hours after something gets going on Twitter, the campaign will next turn to Facebook, to see how it is permeating in the larger universe of public opinion. And by the next day, Google search provides a retrospective of how the whole thing played.

All of that makes the pace of earlier rapid-response efforts — such as the vaunted 1992 Bill Clinton “war room,” or even Obama’s own technology-savvy 2008 effort— seem almost glacial by comparison.

Twitter has also changed the tenor of the debate, with candidates and campaign operatives posting edgy things there that would not likely dare to posit if, say, they were sitting in front of a television camera.

In the past week, for instance, Romney communications director Eric Fehrnstrom noticed a blog post unearthing a passage from Obama’s 1995 autobiography, in which the future president recalled eating dog meat as a child in Indonesia.

Fehrnstrom posted a link to it on Twitter, and it quickly made it onto the Web sites of major news organizations — giving the Romney campaign a rejoinder to the constant tweaking it has gotten over another long-ago episode, in which the former Massachusetts governor transported his own dog to Canada in a crate atop his car.

Indeed, Fehrnstrom’s post included a “retweet” of one months earlier by Obama’s chief strategist David Axelrod, in which Axelrod had posted a picture of the president riding in the back of his limousine with his dog, Bo, as an example of “how loving owners transport their dogs.”

“In hindsight,” Fehrnstrom wrote, “a chilling photo.”

And as a result, that obscure passage from a 17-year-old book suddenly looked newsworthy enough to merit mention in outlets that ranged from the New York Times and Washington Post to ABC News.

These kinds of exchanges — Fehrnstrom and Axelrod have something of a running Twitter feud going — do not exactly represent an elevation of the level of political discourse. Nor is Twitter ever likely to be the place to go if, for instance, a voter is looking for an explanation of the intricacies of clean energy policy.

But the Twitterization of politics also represents a breakthrough, giving ordinary people a chance to be part of the political conversation in a way they never have before.

“With Twitter, we’ve lowered the cost to almost zero for people who also feel passionately to push a point,” said Frank Speiser, the chief executive and co-founder of SocialFlow, a firm that has developed a platform to help its clients use Twitter more effectively.

Twitter was around, but pretty much ignored, during the 2008 presidential campaign, when it was regarded as a pre-occupation of techno-geeks.

And in the early months of the Obama administration, White House officials were leery of it, in no small part because of concerns about how the freewheeling environment of social media would mesh with the requirements of the Presidential Records Act, which mandates that all the traffic end up in the National Archives.

Only when officials noticed how many White House reporters were using it did they realize that “if we weren’t on Twitter, we were missing an opportunity,” said former deputy press secretary Bill Burton, one of the early advocates of setting up official White House Twitter accounts.

While the White House has gradually become more conversational on its official Twitter accounts — befitting the culture of the medium — they say there is nonetheless a different standard required.

Said Jay Carney, known to more than 292,000 Twitter followers as @PressSec: “I treat it like the briefing or anything else I do as press secretary speaking for the president.”

Follow Karen Tumulty on Twitter: @ktumulty