The 37-year-old Leffler, a two-time winner on the NASCAR Nationwide Series who had the nickname "LefTurn" above the driver's side window on his race cars, was pronounced dead Wednesday night shortly after 9 p.m., New Jersey State Police said.
"NASCAR extends its thoughts, prayers and deepest sympathies to the family of Jason Leffler who passed away earlier this evening," NASCAR said in a statement. "For more than a decade, Jason was a fierce competitor in our sport and he will be missed."
Bridgeport Speedway immediately suspended racing for the rest of the night after Leffler's accident at the 0.625-mile, high-banked dirt oval.
After losing his NASCAR ride, Leffler had been racing dirt car events most of this year, including the 410 Sprint Car race Wednesday that promised a $7,000 prize to the winner. On Sunday, Leffler finished last at Pocono in his lone NASCAR Sprint Cup start of the year. He ran just eight laps in a start-and-park ride.
From Long Beach, Calif., Leffler made 423 starts in NASCAR's three national series, but won just the two Nationwide races and one Truck Series event in a career that began in 1999. He also made three IndyCar Series starts, finishing 17th in the 2000 Indianapolis 500.
A statement from Indianapolis Motor Speedway called Leffler "one of the most versatile race drivers in America, showing his talent by competing in the Indianapolis 500 and the Brickyard 400 at the Indianapolis Motor Speedway during his career."
"He also displayed the skills that would help him reach the top levels of the sport by winning four USAC national series titles while winning on tracks throughout the Midwest," the statement said.
Leffler's last full NASCAR season was 2011, when he ran the entire Nationwide schedule for Turner Motorsports. He finished sixth in the standings that season and hadn't had a steady NASCAR ride since.
Although he never made it at the NASCAR Sprint Cup level, Leffler ran almost the entire 2001 season for Chip Ganassi Racing and ran 19 races in 2005 for Joe Gibbs Racing in the car now driven by Denny Hamlin.
Leffler is survived by 5-year-old son Charlie Dean.
"Really sad for Jason Leffler and his family," NASCAR Sprint Cup champion Brad Keselowski said on Twitter. "Thinking about his little boy."
Leffler won three consecutive USAC Midget championships from 1997-99 before following mentor Tony Stewart's path into NASCAR. Stewart-Haas Racing, owned by Stewart, tweeted: "Thoughts and prayers to the family and friends of Jason Leffler. A good man; a good racer. Godspeed friend."
Leffler originally signed with Joe Gibbs Racing, the same team Stewart drove for at the time, and ran the 2000 Nationwide season for JGR. He grabbed four top 10s as a rookie, and moved to Cup the next year with Ganassi in a deal that lasted only one season.
IndyCar driver Justin Wilson tweeted: "Another reminder of how this sport we love can be so cruel. Thoughts with his family." Wilson included the hashtag "LEFturn."
NASCAR driver Clint Bowyer also included the hashtag "LEFturn" in a tweet, saying: "So sad to hear about Jason Leffler. Was a wheel man and a fun fun person to be around."
Many drivers also mentioned Charlie Dean in their tweets. A single father, Leffler was devoted to his son and his Instagram account was full of photos of the two spending time together, including many at race tracks.
"Praying for all of Jason Leffler family, especially his little guy Charlie," tweeted IndyCar driver Ed Carpenter.
A little over two weeks ago, 22-year-old driver Josh Burton died of injuries sustained in a crash at Bloomington Speedway in Indiana. In late May at a dirt track in Nevada, two drivers were killed in a race. In March in California, two people were killed when a car careened off a dirt track and crashed on pit road.
Sprint car races can be more dangerous for drivers and spectators because the safety measures taken by series aren't at the same level. Many facilities lack the SAFER barriers that are standard in NASCAR and IndyCar, and the cars aren't always adequately protected.