Bryce Harper will become the youngest player in National League history to start the all-star game on Tuesday at Citi Field in New York. (Jonathan Newton/The Washington Post)

Bryce Harper will run onto the field at Citi Field in New York on Tuesday for his second all-star game in as many seasons. The Washington Nationals outfielder has played only 195 career major league games and missed 31 games this year with a knee injury, but a fan vote nonetheless made him the youngest position player ever chosen to start for the National League all-star team. He’ll also appear in Monday’s Home Run Derby.

It has reinforced an already apparent truth: Harper is an immensely popular figure, an attention-capturing player fans flock to see on the field and companies want pitching their products off it.

On the field, Harper, 20, has validated the immense hype surrounding him by winning the NL rookie of the year award and sparking the Nationals to their first playoff appearance last season. His is one of the top-selling jerseys in baseball, and he is a featured player in Major League Baseball advertising campaigns. He has appeared in commercials for Toyota, Geico and Under Armour. For the companies he has endorsed and others that want him, Harper is budding marketing gold.

And unlike past baseball prodigies such as Ken Griffey Jr. and Alex Rodriguez, Harper has risen to popularity in a social media world. He was introduced to the world at 16, proclaimed as the sport’s next phenom on the cover of Sports Illustrated in 2009. His response to a reporter’s offbeat question in Toronto last season — “That’s a clown question, bro,” — instantly became a pop-culture catchphrase, used by politicians and emblazoned on a T-shirt. More than 8,400 people jammed into a creaky Pfitzner Stadium in Woodbridge to watch his first minor league rehab game three weeks ago. His signed rookie baseball cards sell for several hundred dollars online.

“He had a cult-type following because of the access that people had to information in real time,” said Matt Mirchin, Under Armour’s senior vice president of brand and sports marketing. “That certainly played a part on his popularity, in his quick rise. But all that goes away if you don’t perform on the field, and he has done an incredible job of coming through the minor league system so quickly, handling his celebrity status as good as he has and performing on the field. That’s what makes him so special right now.”

The Post Sports Live crew debates whether Bryce Harper will have a more exciting appearance in the MLB All-Star Game or in the Home Run Derby. (Post Sports Live)

Harper first caught the attention of Under Armour, his most high-profile sponsor, when he was in high school. Mirchin said the company began talks with Harper once he declared for the 2010 draft, in which the Nationals would select him first overall, and tapped Scott Boras as his adviser. The Baltimore-based company has 175 baseball players under contract, including Buster Posey, Clayton Kershaw, Ryan Zimmerman and Manny Machado, and Harper has become one of its most featured names.

“He’s very marketable for us for a lot of reasons,” Mirchin said. “His energy, we love his style of play, his aggressiveness and he fits the brand exceptionally well.”

Under Armour partnered with 3 Penny Films and MLB Productions to make an hour-long documentary, which aired on ESPN this spring, about Harper’s rapid rise through the minor leagues. Because Harper would be playing left field this season, Under Armour, which already partners with the Nationals, bought ad space for its logo on the left field wall of Nationals Park.

It is not known how much money Harper, who signed a $9.9 million contract with the Nationals in 2010, has earned through endorsements. His endorsement value for 2012 was estimated at $1.2 million, according to an economic model that evaluated 125 popular athletes. Jeff Phillips of the consulting firm the Parthenon Group, who developed the model with Tyler Williams, a graduate student in economics at MIT, said Harper ranked high in the model’s categories of transcendent fame, attractiveness and social media presence.

Harper shies away from talking about endorsements, preferring to focus on what happens on the field. During his rookie season, he dedicated himself to adjusting to the majors and wasn’t seen in ads. While Harper admits it has been fun to be featured in commercials and “pretty cool” to see fans holding up large cutouts of his face, he deflects when asked about his rising popularity.

“Having a lot of guys on our team, [Stephen] Strasburg, [Jayson] Werth, [Ian Desmond], guys that everybody in the league knows about, and us having the good year last year that we did and coming on strong this year, there’s been a lot of popularity with our team and organization,” Harper said. “It’s not just me getting the popularity. It’s Strasburg, [Ryan Zimmerman], Desi, Werth. It’s a mutual thing. I’m really blessed to have fans and people that support me.”

In the second half of last season, Harper’s jersey was the fourth-most popular in baseball, and through the first half of this season, it’s the sixth-highest selling. But while his renown has exploded in a short time, baseball players don’t historically command the endorsement dollars attached to tennis players and golfers, perhaps because it is a team sport. Baseball is also at a disadvantage because, unlike basketball, its equipment can’t be worn in everyday life. In addition, few baseball players reach international fame. Harper’s popularity will depend on his longevity.

The Nationals’ Bryce Harper’s unusual swing has been compared to the likes of baseball legend Babe Ruth. This animation shows just how similar the two players are. (TWP)

“There’s no telling how much he is worth because you don’t know if it’s sustainable or not,” said David Carter, executive director of the University of Southern California Sports Business Institute. “We’ve seen quite a few athletes burst onto the scene that seem to be doing everything right only find themselves out of the league with an injury or something else that goes wrong with their career. It’s very dangerous to say that a player is going to stay on a trajectory that has been established over a season and a half or two seasons.”

Despite his maturity and the support system around him, Harper is still just 20. He loves snacking on Cap’n Crunch, playing with his dog, Swag, and cars. He tweets about his family, sports he watches, motivational quotes and products he likes to use.

Those tweets, of course, can become marketing opportunities. After Harper shared his affinity for Chipotle with his 290,000 Twitter followers earlier this year, the chain gave him a card guaranteeing him free burritos for one year.

Said Chipotle spokesman Chris Arnold in an e-mail: “People like Bryce have tremendous reach.”

And just like many his age, Harper is style-conscious. He gives Under Armour input on his equipment. Other apparel choices, a mix of widely distributed and lesser-known brands of shoes, gloves, bats, sunglasses and more, reflect his taste.

“He’s also an every-kid,” said Duk-ki Yu, who owns Major, a sneaker and lifestyle store in Georgetown, and has served as a creative and marketing consultant for Reebok, Nike, New Era and EA Sports. “. . . He identifies with all the 18- to 25-year-olds out there. You see him in the street, and he’s wearing the same clothes that every 20-year-old kid in college is wearing. Camo shorts, surf company tees, a snap-back [hat]. He is a really like a typical kid that they can really sort of gravitate towards.”