PHILADELPHIA >> President Barack Obama’s re-election campaign is barely a month old, but Camille Gervasio and other volunteers nationwide already are hard at work.

“Are you with us? Are you in?” Gervasio asks into her iPhone, dialing through a call sheet resting on her laptop to line up supporters for an election 18 months away.

In call centers like this one on the eighth floor of an office building, the president’s backers are trying to take advantage of a head start over the still-forming Republican field and the benefits of incumbency to rebuild a grassroots effort that mobilized millions of voters in 2008.

Obama’s campaign has pledged to reach out to every voter it was in contact with during his first run, a herculean 50-state organizational effort to reconnect with its supporters — some of them now disillusioned with the president because of his policies  — while giving it an early indication of any vulnerabilities among critical constituent groups.

Without having to focus on a primary opponent, Obama’s campaign also is spending much of its time and money trying to build foundations of support early in battleground states like Pennsylvania, New Mexico and Ohio that backed Obama last time but have since elected Republican governors, weakening state Democratic Party operations.

“Every single day we have to go scratch and claw for those votes,” Jim Messina, Obama’s campaign manager, said recently in a video outlining Obama’s strategy. He argued that Obama’s team must “act like an insurgent campaign” to win re-election.

In some ways, Obama’s first campaign never folded.

After he was elected, he turned it into an organization called Organizing for America to communicate with supporters, rally them behind his policies and encourage get-out-the-vote efforts during the midterm elections. The group, run through the Democratic National Committee, has been criticized even by some Democrats for being ineffective at translating support for Obama’s campaign into support for his policies.

Since Obama officially announced his re-election campaign in early April, his advisers have been working to reignite the grassroots campaign that was inspired by Obama’s days as a community organizer.

Under the slogan “I’m In!,” volunteer events are under way across the country, from brainstorming sessions at coffee houses to holding phone banks, house parties and door-to-door neighborhood canvassing events.

In Florida on Saturday, Obama volunteers were asked to bring a cell phone and charger to staff phone banks around the state. In Bloomington, Ind., near the campus of Indiana University, the president’s backers are running phone banks twice a week.

New York City supporters held a seminar on using Facebook as an organizing tool. Los Angeles backers entered data collected from thousands of voter commitment cards from an Obama event at the University of Southern California in October 2010. And Las Cruces, N.M., volunteers are running a voter registration drive at a farmer’s market.

“We’ve expanded upon what we did in 2008,” said Jeremy Bird, the Obama campaign’s national field director. “We could have said we’ll come back to you when we run another presidential election, but we’ve maintained contact with our supporters, they’ve been involved in voter outreach, legislative fights, training people.”

Beyond the reintroductions, the early efforts are also aimed at a changed political landscape.

When Obama locked up the Democratic nomination in 2008, he was able to turn to Democratic governors like Ed Rendell of Pennsylvania, Ted Strickland of Ohio and Tim Kaine of Virginia for help.

But, last year, Republicans captured governors’ posts in several states Obama carried in 2008, including Iowa, Michigan, New Mexico, Ohio, Pennsylvania and Wisconsin. And in 2009, Virginia voters elected Republican Bob McDonnell to succeed Kaine. Collectively, the seven states now account for 88 electoral votes, or nearly a third of the 270 electoral votes needed to win the White House.

Controlling a state’s top political office can be a boon for a party’s White House nominee because governors frequently offer up efficient campaign operations, a long list of financial supporters and key advisers to a presidential campaign. Governors also are frequent surrogates for presidential campaigns and can mold their state’s get-out-the-vote operation to aid the top of the ticket.

Republicans question whether Obama’s grassroots operation will be as formidable this time.

“In 2008 they were able to create an organizational juggernaut because people were inspired by the Obama candidacy. But I think when you look at his record going into 2012, especially with independents, there may be less inspiration, which makes the organizing more difficult,” said Republican strategist Jim Dyke, a veteran of presidential politics.

But Democrats say recent battles over labor unions and state budgets in Ohio and Wisconsin, where Republican Gov. Scott Walker has sought to curtail collective-bargaining rights for public employees, has energized its voters and could help them next year.

“The Democratic base, which may have been lukewarm about the president … is now fired up,” said Rendell, the former Pennsylvania governor. He said several Rust Belt states “have seen this incredible reversal of Republican fortunes in 2011 because of the mistakes and wrongheaded policies.”

Overtly or not, it’s a message Obama backers are sending as they encourage others to join their ranks.

In Philadelphia’s Center City, about 20 volunteers sat at plastic folding tables one night last week, manning phones near two boxes of Dunkin’ Donuts. With campaign posters and photos of Obama on the walls, volunteers called local supporters from 2008, asking them to help again.

A script they read says: “It is people like us — working together, reaching out to others to grow the movement — that will not only help re-elect the president and other Democrats but also help move America forward.”

If a past supporter responded negatively, the caller asked if there was “something holding you back,” and noted whether it pertained to issues such as education, the environment, foreign policy, health care or jobs.

“This is laying the groundwork,” said volunteer Sam Jones, who last worked on a presidential campaign as a college student for George McGovern in 1972.

Gervasio, who worked on Wall Street and taught math in Philadelphia before semiretiring, knocked on doors in 2008 but only recently signed up to help again after getting a pitch from a volunteer leader looking to build a neighborhood team.

“Now,” she said, “I’m here trying to find more.”