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The gains that have stimulated businesses and revitalized neighborhoods have been Mr. Gray’s principle defense against criticism over the Thompson investigation, which included guilty pleas of four other Gray aides and associates in connection with the shadow campaign.

“People want to hear about the future of the District of Columbia. They want to hear about what we are going to do about education, what we are going to do to assure the fiscal stability of the city,” Mr. Gray said in the first public debate after Thompson’s guilty plea.

Since then, the mayor has doubled down his efforts to secure votes from his base: black voters from the city’s poorer wards east of the Anacostia River that were crucial to his 2010 win. The maneuvering, including a prominent endorsement from four-time mayor and D.C. Council member Marion Barry, has left some to question Mr. Gray’s commitment to his own “One City” motto that promised to unite the District.

It remains to be seen whether any of the Democrats in the field can capture the imagination of voters and persuade them to fire a chief executive who leads one of the most prosperous cities in the nation.

“We have a plan. It’s working,” Mr. Gray says in a video snippet used at the end of his campaign commercials. “And that’s why I’m proud to be your mayor.”