Stephen Lam / Getty Images President Barack Obama speaks about the Affordable Care Act at the Fairmont Hotel in San Jose, Calif., on June 6, 2013

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In a windowless office in the basement of the West Wing, behind a utility room and next to a fire-alarm panel, David Simas is marking up another whiteboard with blue ink, hoping to explain one more time just how the President’s health care reform law will work when the time comes for uninusured Americans to sign up this fall.

The abstraction of Obamacare is about to become reality. But will anyone know what to do? An estimated 7 million Americans will join the first health exchanges from Oct. 1, 2013, to April 1, 2014, and most have no idea how to do that. And for the millions who are already covered by some early Obamacare benefits, Simas adds, “We need to continue to remind them of what they’re getting.”

If Simas, 42, has the nervous energy of the 2012 campaign, where he served as opinion-research director, there is a good reason. Though Barack Obama has competed in his last election, he has one more campaign ahead of him. This fall, Obamacare will go into full effect, with the promise of insuring as many as 40 million Americans if it succeeds. But it could still fail.

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To prevent that, Obama-land is going on offense. Organizing for Action, the grassroots group spun out of the President’s campaign, has made selling the already-passed bill a top priority with its first television ad. Enroll America, a nonprofit coalition of community groups and insurers that has been promoted by the White House and is staffed by Obama campaign alumni, launched its “Get Covered America” campaign to educate uninsured Americans about the exchanges.

At the White House, health care implementation has become an obsession. Chief of Staff Denis McDonough spends two hours a day on Obamacare implementation, staffers said, and senior aides like Simas and Tara McGuinness, who joined the White House in April as a senior communications adviser, work on the issue nearly full-time. Hardly a week goes by without Obama finding some way to plug the effort as well.

The reason: the law is increasingly unpopular. According to a NBC News–Wall Street Journal poll released earlier this month, 49% of Americans now believe the law is a bad idea, the highest percentage recorded, with only 37% saying it is a good thing. Many states have already opted out of key provisions to expand Medicaid. In Washington, Republicans continue to lay siege to the law; they have voted to repeal it 37 times in the U.S. House.

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That unpopularity threatens one of the law’s most ambitious goals—establishing health care exchanges allowing uninsured Americans to purchase affordable coverage. The exchanges need roughly 2.7 million healthy 18-t0-35-year-olds to sign up to be solvent. The majority of that group is nonwhite and male, according to Simas’ data, and a third are located in just three states: California, Texas and Florida. If too few choose to enroll because they don’t know about the law, don’t like it, or feel they don’t need insurance, the exchanges will fail. And so will the law.

The Administration has plotted an extensive social-media campaign designed to reach the young and healthy and is soliciting sports teams to help raise awareness. More than 10 staffers in the Office of Public Engagement are marshalling the help of Latino and African American groups and community nonprofits. And Simas has spent countless hours surrounded by maps of media markets and demographic data on the uninsured trying to remind prospective enrollees of the benefits available to them: “It’s that guy in Dallas, it’s the woman in Los Angeles, it’s the family in Miami-Dade,” he says.

Obama aides have been saying such things for years, with little effect. But now the clock is ticking, and Obama is running out of time to make his signature achievement stick.