Also click here to view a timeline of the often raucous greetings received by world leaders at the International AIDS Conference.

After a 22-year hiatus, the International AIDS Conference will return to the United States next week, gathering more than 25,000 participants in Washington, D.C. President Barack Obama has a standing invitation to speak at the mega-gathering, and he likely would be welcomed with loud cheers given his progressive HIV/AIDS policies, including lifting a congressionally mandated immigration ban of infected people that led the meeting organizers to shun the country for two decades. Many world leaders have addressed past opening ceremonies in person to welcome the attendees to their countries. But Obama apparently can’t carve out the time, which both runs the risk of angering a volatile community and squandering a historic opportunity.

The meeting, now held every other year, does not resemble any other scientific conference. The attendees of AIDS 2012 include scientists, clinicians, political leaders, movie stars, business moguls, HIV-infected people, and representatives from particularly hard-hit communities (gays and transgenders, sex workers, injecting drug users, African Americans, sub-Saharan Africans, Latinos). Among the notable participants this year—theme: “Turn the Tide Together”—are Bill Gates, Whoopi Goldberg, Elton John, Laura Bush, and former President Bill Clinton. A few thousand people from the media typically show up, filling the press room with so many journalists that the profession actually seems alive and well. The proceedings mix presentations of new research findings with a splash of Woodstock, a dollop of heart-wrenching and fiery talks from infected people, and spicy activism that traditionally floods the streets and even the conference sessions themselves with protests.

No sitting president attended the three previous meetings held in the United States, but those were different times. Ronald Reagan was president when Atlanta hosted the first International AIDS Conference in 1985, and his administration disdained affected communities as well as the scientific findings (on the benefits of syringe exchange and condom promotion) that did not fit with the conservative canon of “just say no” to drugs and sex. As AIDS activism exploded in the late 1980s, that U.S.-government disdain was the target of high profile protests—featured in the new documentary How To Survive a Plague—that shut down the National Institutes of Health, the Food and Drug Administration, and the New York Stock Exchange. At the last international AIDS conference held in the country, the 1990 meeting in San Francisco, activists stopped downtown traffic to protest the U.S. immigration policy that barred HIV-infected people. U.S. government officials who have made presentations at the meeting, including Vice President George H.W. Bush and health secretaries Louis Sullivan and Tommy Thompson, have weathered humiliating greetings. Hecklers booed the vice president and made such a din with bullhorns and whistles when the health secretaries appeared on stage that no one could hear their speeches.

But Obama would face none of this hostility. The United States today spends more money on HIV/AIDS research than all countries combined and also is the single most generous donor to the global effort to combat the disease. In 1996, effective antiretroviral drugs finally came to market and “access for all” became the mantra, which, in a startling move, none other than President George W. Bush embraced. In what many see as Bush’s greatest achievement while in the White House, he created the President’s Emergency Plan for AIDS Relief in 2003 and also backed the newly formed Global Fund To Fight AIDS, Tuberculosis, and Malaria, two progams that together now provide treatment to 8 million HIV-infected people living outside this country. Domestically, Obama has received high praise for issuing the first national plan to combat the disease and shuffling money to help the most affected populations. He also has championed the scientific evidence that antiretroviral treatment is a form of prevention—the drugs make people less infectious—and argued for expanding treatment to create what he called in a World AIDS Day speech last December “an AIDS-free generation.”

Obama is often called upon to participate in events in part because he is a symbol of progress and history, but his background has an especially relevant resonance with HIV/AIDS. As a black American, Obama belongs to a community that makes up 14 percent of the population but now accounts for nearly half of the new infections in this country. He also is the son of a Kenyan, and no region has suffered more from HIV than sub-Saharan Africa.

Who knows why Obama didn’t want to make the short drive from the White House to the conference center. In May, I put the question to Grant Colfax, who heads the White House Office of National AIDS Policy. “Obviously he’s very aware of the conference, and at this point it’s too early to comment on the president’s schedule,” said Colfax. On July 16, the White House issued a press release, “Obama Administration To Participate in the 19^th International AIDS Conference,” that listed all the top officials who will attend—Secretary of State Hillary Clinton among them—but one name was conspicuously absent. “Continuing his personal engagement on this issue, the President will provide a brief video message to welcome Conference attendees from around the world to Washington,” the statement says. White House spokesperson Shin Inouye confirmed that the video is in lieu of his appearing. “The President will not be speaking at the conference,” Inouye wrote in an email to me.

Later in that afternoon, the meeting organizers said Obama had yet to decline their invitation to speak in person. But I’d save my betting money.

Maybe Obama’s political calculus in this election year is that attending the meeting has no upside when it comes to winning votes: He has the support of blacks and gays, the populations most affected by HIV/AIDS, and there could be a backlash if he emphasizes his support for the world’s poor and downtrodden, men who have sex with men, drug users, sex workers, and other marginalized communities. Maybe his handlers are afraid that activists will target him for not doing more. Maybe it’s a concern about security. Maybe he has a scheduling conflict like the one that kept him from attending a meeting of the NAACP last week.

If Obama presence is limited to a video welcoming, activists surely will hurl some shouts of “Shame! Shame! Shame!” his way. But aside from turning allies into adversaries and having to endure some unflattering imagery on the evening news, he also will be turning his back on a chance to fortify his position as the leader of the global movement to end the epidemic. Yes, it could cost him votes. But it could also win him the world’s lasting admiration and gratitude.