When the Philadelphia Union unveiled its blue-and-gold soccer jersey, the shirt contained one element that team officials plan to change:

The big blank spot on the chest.

The Union's executives want to fill that space with the logo of a high-paying corporate sponsor.

Panasonic would be good. Comcast, maybe even better.

Putting ads on the venerable jerseys of pro baseball, football, and basketball teams may be anathema to U.S. sports fans, but it's common practice among overseas soccer clubs, which reap millions of dollars in revenue. The deals are particularly vital to Major League Soccer, which after 14 seasons still fights for attention, advertising, and fans.

The league's attendance dropped 2.6 percent this year, and without the blinding success of the expansion Seattle Sounders - who drew an average of 30,897 fans a game - it would have fallen 9 percent. The recession is making teams such as the Union, which will take the field in March, battle for every dollar.

Union president Tom Veit said the money that will come from a jersey sponsor was important. But more crucial than getting a sponsor, he said, is getting the right sponsor.

"One of the most sacred things we have is our jersey, and that brand is going to be there," he said. "We've got to be very careful, and very diligent, about whom we're going to put on there. . . . No other sponsor, even stadium naming rights, is going to be associated with the [team] brand at a higher level."

The Union heard early on from a couple of interested companies, but team executives didn't see either as a good fit, he said. Union officials are now talking to two firms that seem like better candidates, though it's unclear how those discussions will conclude, Veit said.

In European and other countries, front-of-the-jersey advertising is the norm, the logos of airlines, paint companies, and electronics-makers sitting cheek-by-jowl with club crests. Fans view the ad as part of the uniform. If they buy the red jersey of Liverpool FC, it had better say "Carlsberg" on the chest.

To advertisers, the value of that association is enormous - which is why they pay dearly for the privilege.

In June, Chicago-based insurance broker Aon Corp. signed a $130 million, four-year deal to sponsor Manchester United, starting with the 2010-11 season.

What does Aon get for its money? Lots.

United's fanatical worldwide following makes the front of its shirt some of the most valuable space in advertising. Every time a player is photographed, videotaped, or televised, Aon will piggyback on that image, its name and logo appearing in countless newspapers, magazines, Web sites, broadcasts, and highlight reels.

Second, Manchester United sells about six million jerseys a year - each to bear the Aon logo, spreading the brand.

Third, and not least, the firm aligns itself with one of the marquee franchises in sports. The association creates the inference that if Manchester United is a winner, and it surely is, then Aon is a winner, too.

The lure of that revenue, and the pain of the recession, is prodding major U.S. sports leagues to consider whether they should accept shirt sponsors - and how fans would react.

This season, the NBA allowed teams to sell ads on their practice jerseys, and the league is exploring having ads on game jerseys. The NHL, which already sells ads on the boards of its rinks, says it might put sponsors on jerseys. The NFL recently changed its rules to allow ads on practice jerseys, and several teams have generated income that way. The New York Jets, for instance, get more than $2 million a year from Atlantic Health.

But to many fans, the game-day jersey is sacrosanct, inseparably bound to past and future glory. Imagine the reaction if the Phillies added "Hatfield Meats" to their red pinstripes, or the Eagles took the field wearing the red, white, and blue circle of PepsiCo.

"It's really sacred territory," said Liz Panich, director of consulting for the Marketing Arm, a Chicago-based promotions firm. Still, she said, "I think one of the leagues will take a chance on it and see how it works."

Major League Baseball got a hint of what could go wrong in 2004, when it announced plans to promote Spider-Man 2 by putting the movie's logo on bases. The outcry forced officials to cancel the promotion a day later.

But no such psychological wall exists in world soccer, and certainly not in the MLS. Since the league began to allow the agreements in 2007, 11 of the 15 teams have signed deals.

The MLS set a $500,000 minimum for shirt sponsorships, though clubs generally earn more.

A small-market team such as Real Salt Lake, which won the MLS Cup last month, is on the low end, with a package worth about $1 million. At the high end, Herbalife paid about $5 million to sponsor the David Beckham-led L.A. Galaxy, which lost the Cup game on penalty kicks.

The Seattle Sounders, sponsored by Microsoft, wear jerseys that say "Xbox 360 Live." The Columbus Crew is sponsored by Glidden, Toronto FC by Bank of Montreal.

In January, the San Jose Earthquakes signed a three-year deal with Amway Global, reported by Reuters to be worth $2 million to $3 million a year.

That's critical income to a team that lost $2.2 million in its first season, in 2008, and expected to lose money this year. For Amway, the deal offered a connection to a family-friendly sport and a chance to reach the city's large Hispanic population, traditionally soccer enthusiasts.

Panich, of the Marketing Arm, said the key to a successful sponsorship was for team and company to fit, for each to be proud, or at least accepting, of the other. No team wants to be sponsored by Acme Toxic Waste, at any price, and no company wants to be tied to a poorly run, perennially losing club.

Panich expects the Union to land a midlevel deal in the range of $2 million to $3 million, similar to those of the Chicago Fire, sponsored by Best Buy, and DC United, sponsored by Volkswagen.

The Union revealed its official jersey in a rush last month, after an image had leaked onto a vendor's Internet site.

One of the first questions from fans was whether to buy right away or wait until the Union had a sponsor. The team promised that anyone who bought a "blank" jersey could have the sponsor added later.

Overall, fans' reaction to the shirt - with its thick, vertical gold stripe down the chest - has been mixed.

"Is it me, or does Philadelphia Union's soccer jersey look very similar to the Shrewsbury Town jersey worn by Derek Smalls in the cult classic movie This Is Spinal Tap?" one blogger wrote.

Others suggested it looked like a highway dividing line.

Plenty of fans thought the Union shirt looked great - bold, different from other teams but still classic, a design that won't need to be tweaked every five years.

The team already has deals with two muscular companies that could step up to jersey sponsorship. Comcast-Spectacor is to manage the Union's concessions, catering, and ticket sales, and Panasonic will provide broadcast systems and large-screen LED displays.

Veit, the team president, said that because the Union was new, the sponsorship decision takes on added weight. The name on the shirt will help mold perceptions of the team.

"We're looking at some national brands and regional brands," he said. "The sponsor we sign, we want to have for a long time."