Emails suggesting that close aides to New Jersey Gov. Chris Christie engineered traffic jams as political retribution against a Democratic mayor are turning into a major test for the national ambitions of a popular Republican considered a leading 2016 presidential contender.

On Wednesday, the newly released emails showed that a top Christie aide suggested that another longtime associate of the governor create "traffic problems" in Fort Lee, N.J. Weeks later, the second official imposed lane closures leading to the...