President Barack Obama is turning 51 on Saturday, with just three months until Election Day. So he will celebrate in true political fashion: spending “downtime,” as an email invitation put it, at a party at his Chicago home with a bunch of strangers who made campaign donations to be there.

That party, on Aug. 12, will mark another milestone in the transformation of the president and his wife, who once tried to limit the role of politics in their lives and now seem to be increasingly giving themselves over to it. Even some longtime Obama fundraisers expressed surprise over the party’s site: The Obamas have limited their schmoozing hours in Washington, sequester themselves while on vacation and seldom invite many outsiders into the White House living quarters. Until now, they have kept their Chicago home mostly sacrosanct, allowing only limited photographs of the interior.

The party raises questions about how far the Obamas will go in mortgaging their personal appeal for political gain in the months ahead. In poll after poll, voters give Obama higher marks as a person — a trustworthy leader, a committed father — than as a steward of the economy. Aside from their house, how much of themselves are the Obamas willing to offer up?

Consider the shift in Obama’s birthday celebrations since he arrived in office in 2009. That year, with re-election still distant, Michelle Obama threw him an entirely private surprise party at Camp David, Md., with a few old friends. His 2010 party was splashier, a basketball tournament that made news because of the professional and college stars who played.

A year later, he celebrated his 50th birthday with a fundraising gala at Chicago’s Aragon Ballroom, where Jennifer Hudson sang “Happy Birthday.” To scrape up extra dollars, the Obama campaign sold commemorative birthday merchandise, including party hats. Even the private party he threw at the White House mixed old friends with allies who might assist in his re-election fight, like Tom Hanks and the Rev. Al Sharpton.

Every year, some Obama supporters have gathered all over the country to “celebrate” his birthday — that is, to share his message and sign up converts. But in the coming days, they will hold no fewer than 1,000 birthday-themed events, from North Carolina (knocking on 51 doors) to California (calling 51 voters).

Birthday fundraisers are a familiar tactic that may have reached an apotheosis in 1962, when Marilyn Monroe sang to President John F. Kennedy at a Madison Square Garden rally attended by 15,000 people and broadcast on television. In 1996, President Bill Clinton filled Radio City Music Hall for a 50th-birthday fundraiser that brought in $10 million.

But the one at the Obamas’ home will have a specific 2012 spin, reflecting the illusion of intimacy on which campaigns now thrive. In recent years, win-a-visit-with-Barack (or Mitt or Ann or Joe, or Sarah Jessica Parker or Marc Anthony) sweepstakes have become a signal fundraising tactic for both sides. A recent Obama event at George Clooney’s home demonstrates why: The campaign collected $15 million, according to organizers. Less than half of that came from the Hollywood types who paid $40,000 a ticket; the rest came from a sea of supporters who made small donations and entered an online contest to win seats.

The formula has proved so lucrative that Mitt Romney’s campaign advertised the chance to be introduced to Romney and his vice presidential nominee: A contest to meet a political partner who does not yet exist. Aside from harvesting new email addresses, the contests allow the campaigns to “drive their small-dollar contributions off their big-donor contributions,” said Anthony Corrado, a campaign finance expert at Colby College.

Months after winning the Clooney contest, Karen Blutcher, 45, a communications manager for a utility company in St. Augustine, Fla., still sounded thunderstruck that she had met the president and mingled with stars. “The entire event was a lifelong memory,” she said in an interview.

A warning to this month’s lucky winner: The celebration at the Obama home will be considerably less cozy than the emailed invitations suggest. The event is not a social gathering; it is one of four fundraisers that Obama will race through that day in Chicago. Despite a gracious invitation that the first lady emailed to supporters, she is not planning to attend, a campaign official said.

Donors may dream of tinkling the keys on the family piano or leafing through private photos, but much of the action will take place in the backyard. The finalists will be chosen at random, but the contest winners will be selected as reality show contestants are, to make sure their stories fit. (Blutcher comes from a hotly contested state and supports Obama’s health care overhaul because she has a son with special needs.)

Even the promise of being allowed in the Obamas’ house is a bit of a mirage: The first family rarely spends nights there anymore, and many Chicago friends predict they will never move back in.

Also — and the campaign does not play up this bit of fine print — but charging money to enter contests is illegal. So the legions of contestants who reached into their own pockets may not know it, but they could have entered for free.

© 2012 The New York Times Company