(Adds new voter quote, details on turnout)
By Ian Simpson and Tom Ramstack
WASHINGTON, April 1 (Reuters) - District of Columbia residents voted on Tuesday in a crowded Democratic mayoral primary, with the scandal-tarnished incumbent, Vincent Gray, facing a stiff challenge from a city councilor.
Polls show Muriel Bowser, who helped manage Adrian Fenty's successful 2006 mayoral campaign and later was elected to the city council, in a tight race with Gray, who has presided over an economic boom in the U.S. capital.
Winning the eight-candidate Democratic primary is seen as tantamount to taking the general election in a city that is heavily Democratic.
Gray's re-election campaign has been dogged by questions stemming from the financing of his 2010 primary victory over Fenty. Three of his former officials have pled guilty to federal campaign financing charges.
The 71-year-old Gray has denied any wrongdoing, but many voters said the scandal had dulled their view of him.
"D.C. doesn't need any more trouble, so we certainly don't need an indicted mayor," Cheree Cleghorn said shortly after casting her vote in northwest Washington.
Bowser has been endorsed by the Washington Post and enjoyed a funding advantage during the campaign. She had raised $1.4 million to Gray's $1.2 million as of March 24, according to the District of Columbia's Office of Campaign Finance.
Bowser had 30 percent of likely voters' support versus 27 percent for Gray in a Post poll released last week.
There were no official estimates of the turnout by mid-afternoon, but a number of polling places had short or no lines.
A low turnout could help Gray who is expected to have more dedicated supporters than Bowser. The number of ballots cast in early voting leading up to the election was about two-thirds that of the 2010 primary, an election board spokeswoman said.
Gray has seen his lead over Bowser in polls vanish since Washington businessman Jeffrey Thompson, a former government contractor, pleaded guilty three weeks ago to violating campaign finance laws.
Federal prosecutors had accused Thompson of aiding Gray through a "shadow campaign" that funneled more than $660,000 through friends and relatives to Gray's 2010 election effort.
Gray, a former head of the city's human services department, has denied anything illegal took place.
Thompson's plea bargain was the latest in a long line of District of Columbia scandals that include the 2008 conviction of a tax official for embezzling almost $50 million and Mayor Marion Barry's drug conviction in 1990.
The winner of the primary will face David Catania, an openly gay independent member of the city council who is seen as the strongest challenger in November. (Additional reporting by Lacey Johnson; Editing by Scott Malone, Leslie Adler and Paul Simao)