John S. Wisniewski, the state lawmaker who helped expose the George Washington Bridge lane-closure scandal that imperiled Gov. Christie's political career, announced Tuesday that he would run for governor of New Jersey.

Wisniewski (D., Middlesex), 54, who has served in the Assembly since 1996, faces an extraordinary challenge in the Democratic primary in June.

Phil Murphy, a former executive with the investment bank Goldman Sachs and ambassador to Germany under President Obama, has emerged as the clear front-runner.

Murphy, 59, of Monmouth County, has been endorsed by most of the state's Democratic leaders, public-sector unions, and other groups.

Murphy launched his candidacy in the spring and immediately donated $10 million to his campaign. He is the only other declared Democratic candidate.

Republican contenders include Assemblyman Jack Ciattarelli of Somerset County, the only declared GOP candidate, and Lt. Gov. Kim Guadagno.

Wisniewski on Tuesday sought to draw a clear contrast with his Democratic opponent. "I'm not a Wall Street executive. I haven't made hundreds of millions of dollars by outsourcing jobs," he said in a statement.

"I've spent 21 years in the state Assembly fighting entrenched special interests that want to run New Jersey for themselves," said Wisniewski, who was state director for Vermont Sen. Bernie Sanders' presidential campaign.

"That's why I wasn't afraid to be the lead investigator for Bridgegate, even when some Democratic leaders told me it would be an embarrassment."

As chairman of the Assembly Transportation Committee, Wisniewski in 2013 held hearings on the lane closures and served a subpoena on David Wildstein, a high-ranking Christie ally at the Port Authority of New York and New Jersey.

Wildstein turned over documents related to the September 2013 lane closures, including an email exchange between him and Bridget Anne Kelly, then Christie's deputy chief of staff.

"Time for some traffic problems in Fort Lee," she wrote Wildstein in an email a month before the closures and resulting traffic jams near the bridge. "Got it," Wildstein replied.

Kelly and another Christie ally were convicted this month of intentionally misusing Port Authority resources, wire fraud, civil rights violations, and related conspiracy counts.

Prosecutors accused Kelly, Port Authority official Bill Baroni, and Wildstein of conspiring to close access lanes to the bridge to punish Mayor Mark Sokolich of Fort Lee, Bergen County.

Kelly and Baroni have vowed to appeal.

Christie's second term ends in January 2018.

856-779-3846 @AndrewSeidman