Early voting begins Monday in one of the most roiled elections in D.C. history, with a federal investigation encircling Mayor Vincent C. Gray and questions swirling about the magnitude of voter turnout in the city’s first mayoral primary to be held in April.

“To say it’s a fluid situation would be an understatement,” said D.C. Council Chairman Phil Mendelson. “I don’t know that anyone knows how it will turn out.”

The voting for the April 1 primary will start just seven days after federal prosecutors said that Gray (D) knew of a “shadow campaign” waged on his behalf to win in 2010. They also said that Gray personally solicited more than $400,000 for the effort from Jeffrey E. Thompson, a city contractor, and that Gray referred to him as “Uncle Earl” to keep Thompson’s role secret.

Last week, Gray fiercely denied all but the “Uncle Earl” part, saying Thompson wanted to use the alias to keep his support for Gray hidden from then-Mayor Adrian M. Fenty (D). The rest, Gray said, was “lies.” Gray said he never asked Thompson for money to be kept off the books or even knew that such funds were being spent. But the fast-moving developments added fire to a race that for months had appeared nearly frozen.

The city’s first mayoral primary to be held in April — instead of the usual September — meant that candidates spent as much time in recent months educating voters that a race was taking place as they did trying to persuade them to oust an incumbent in a city with a booming economy.

In a crowded field, polls consistently showed Gray leading in a race devoid of the summer barbecues, parades and festivals that typically provide mile markers for mayoral campaigns.

Rather, in one of the coldest winters in years, most days started or ended with challengers standing in freezing weather appealing for votes. Often, their arms extended for handshakes were left hanging as potential voters rushed past, hands thrust into pockets for warmth.

Christmas and New Year’s holidays sent campaigns into hibernation at the equivalent point four years ago when Gray was gaining momentum and surprised Fenty by passing him in fundraising.

This time, there was no such surge.

Council member Muriel Bowser (D-Ward 4) won two nonbinding straw polls in wards early in the year, but it remained unclear whether most voters were even tuned in. Winter storms canceled fundraisers and forums.

Even for Monday’s first day of early voting, the city was under yet another winter storm watch. With a threat of snow, several candidates on Saturday were assessing whether to delay plans to send busloads of supporters to the polls as well as to even cast their own first ceremonial votes.

“There’s no question: The calendar has made it extremely difficult,” said council member Tommy Wells (D-Ward 6), another challenger to Gray. Wells, like Bowser, started knocking on doors last summer to try to make contact with potential voters before the weather became a factor. But the early effort was met with halting success.

“They’d look at us and say, ‘What are you campaigning for?’ ” Wells said.

The District’s early primary is mostly attributable to the Military and Overseas Voter Empowerment (MOVE) Act, passed by Congress in 2009.

The law sought to ensure that soldiers and other U.S. citizens overseas have the same right to vote as those living at home. It stipulated that ballots be sent to voters overseas at least 45 days before federal elections, a timeline that could not be met with the city’s old September primary schedule.

To comply with the law, the D.C. Council voted in 2011 to move the primary to the first Tuesday in April, reasoning the early spring date would allow the city to hold its primary early enough in the year to also be relevant in the presidential campaign.

The city’s first April primary was held in 2012. The reviews were almost universally dismal. President Obama got about half the votes he had four years earlier. In all, a little more than 64,000 of roughly 379,000 registered voters cast ballots — a turnout of less than 17 percent.

Nearly 138,000 voted in the September 2010 mayoral primary.

The earlier cycle was harder on challengers, many believed, leading officials, including Mendelson and D.C. Attorney General Irvin B. Nathan, to refer to the law that set the early election date as the “incumbent protection” act.

Mendelson sought unsuccessfully to move the primary to June this year, saying it would still leave enough time to send ballots overseas while also giving people more time to become interested in the election and give challengers enough time to campaign effectively.

His effort failed. Eight candidates qualified for the ballot. Among them were four council members — Jack Evans (Ward 2), Vincent B. Orange (At Large), Bowser and Wells — as well as former State Department official Reta Jo Lewis, restaurateur Andy Shallal and musician/promoter Carlos Allen.

Gray was the final entrant into the race on Dec. 2, and the crowded field and shortened campaign season appeared to be working in his favor until last week.

The set-up seemed to echo the District’s crowded primary field in 1998, when then-Chief Financial Officer Anthony A. Williams beat three D.C. Council members for the Democratic nomination.

In a Washington Post poll in January, Gray appeared in command. While nearly three in four Democratic voters said the federal investigation would play at least some role in their choice for mayor, respondents gave the mayor high marks overall. He led his closest challenger, Bowser, by double digits.

However, with last week’s developments, polls are set to open with a cloud of legal uncertainty hanging over Gray.

At an event Saturday in Anacostia, the mayor remained upbeat, ticking off a list of plans. “And we’re gonna have a second term. . . . We’re gonna keep the good times rolling,” Gray said.

Beginning Monday, only polls at One Judiciary Square, 441 Fourth St. NW, will be open for early voting. Beginning March 22, 12 more sites will open citywide.

Early voting locations are open daily from 8:30 a.m. to 7 p.m. except on March 23.

The D.C. Board of Elections is also experimenting this year with a tool to give voters a sense of how long they can expect to wait in line before they leave for an early-
polling center. The feature, called “The Queue,” will be accessible on the board’s Web site.

Mike DeBonis contributed to this report.