Democrats remained on course to take control of the Virginia Senate after winning a key special election Tuesday, as thousands of Northern Virginia voters braved snow and bitter winds to cast ballots in an unusual, three-way contest.

In the race to fill the seat vacated by Attorney General Mark R. Herring (D), Democrat Jennifer Wexton prevailed over Republican John Whitbeck and independent Joe T. May, a former Republican delegate running as an independent, according to unofficial election results. The district encompasses a slice of Fairfax County and a hefty portion of eastern Loudoun County, a region that has leaned toward Democrats in recent elections but remains battleground territory.

With the Virginia Senate previously split 20-20, Democrats must hold the two seats vacated by Herring and Lt. Gov. Ralph S. Northam (D) so they don’t lose control to Republicans. If the chamber remains evenly divided, Northam would act as a tie-breaking vote, giving Democrats control of the chamber. Republicans had previously controlled the chamber with a GOP lieutenant governor, Bill Bolling, providing the tie-breaking vote.

The race to replace Northam in his former Senate district, which is based in Norfolk and also leans Democratic, remains undecided. Del. Lynwood W. Lewis (D-Accomack) was certified the winner of a special election by just nine votes, prompting Republican Wayne Coleman on Thursday to request a recount.

With so much at stake, both parties focused intensely on Tuesday’s contest. Wexton received 53 percent of the vote to Whitbeck’s 38 percent and May’s less than 10 percent, according to the State Board of Elections.

For Republicans, the race marked a chance to seize complete control of the General Assembly, where they already hold the House of Delegates by a wide margin. Their goal was to obtain more-even political footing with new Gov. Terry McAuliffe, as the Democrat pursues an agenda, particularly the expansion of Medicaid, that Republicans strongly oppose.

For Democrats, the campaign was a fresh test of whether their vaunted turnout operation, which benefited McAuliffe and President Obama, could be effective in smaller races. Obama won 59 percent of the vote in the 33rd District in 2012, so a loss Tuesday of the seat — and the Senate — would have been embarrassing for Democrats.

Even without snow, Republicans have historically benefited from low-turnout elections through strong grass-roots efforts to get voters to polls. Whitbeck’s campaign said Tuesday that about 100 volunteers were busy making calls on behalf of the Republican candidate.

But his opponents appeared determined to put up a considerable fight: Wexton’s campaign said it had mobilized about 200 volunteers to make calls, knock on doors and hand out campaign literature. May’s campaign had a 20-person phone bank operating throughout the day, and campaign volunteers were stationed at polls across the district, according to May campaign spokesman Jon Conradi.

The election followed a short but highly charged race between the three contenders. Wexton focused on motivating the Democratic base, particularly women, while Whitbeck targeted conservatives by accusing May and Wexton of being liberal “peas in a pod.” With less than 10 percent of the vote, May, who served in the House of Delegates for 20 years, fell far short of his goal of splitting the difference by attracting independents and moderates from both parties.

“United beats divided every time in politics — even in the snow,” said Stephen Farnsworth, a political science professor at the University of Mary Washington. “Republicans need to be more unified — and more moderate — if they expect to win swing districts in Northern Virginia.”

Wexton was helped by her major fundraising advantage: She took in more than $900,000 in the time-shortened race, with roughly two-thirds of the cash coming from Virginia Democratic Party committees and party leaders. Whitbeck raised less than $400,000, and May took in less than $250,000 — the bulk of it from his own pocket.

“We’re elated,” Senate Democratic leader Richard L. Saslaw (Fairfax) said of Wexton’s win. “She’s extremely bright, and she’s going to make a great addition to the Senate.” Tuesday’s off-cycle election was expected to produce a turnout of about 15 percent, according to election officials — a modest estimate that came before the forecast of a significant winter storm that shuttered schools and closed the federal government.

But the severe weather did not appear to deter voters. Election officials in Loudoun and Fairfax reported a higher than 16 percent turnout by 5 p.m. Tuesday. Some precincts in eastern Loudoun and parts of Fairfax saw turnout as high as 20 percent, officials said.

“The fact that we’re almost to 17 percent on a snow day, that’s pretty good,” said Brian Schoeneman, secretary of the Fairfax Board of Elections. “I’m glad to see people turning out and recognizing the importance of the race.”

Still more residents of Loudoun and Fairfax were turned away from the polls because they lived outside the 33rd Senate District, officials said. Election officials in both counties said there was widespread confusion about who was eligible to vote in the race.

Loudoun Registrar Judy Brown said the problem was exacerbated in western Loudoun, where many residents were previously represented by May, who served as a Republican delegate in Virginia’s 33rd House District for more than 20 years before he was ousted in a primary in June.

“When people see or hear [May’s] name, they believe they should be voting,” Brown said. “Those people out west are showing up at voting places along with everyone else.”

Brown said she had also heard reports that campaign calls and mailings reached some residents who lived outside the district.

Leesburg resident Steve Robin, 76, said the snow was already coming down steadily when he voted with his wife about 8:30 a.m. at Smarts Mill Middle School in Leesburg. Robin, who said he generally votes for Democrats, said he and his wife both voted for May because of his experience and reputation in the General Assembly.

“He’s been in the House so long that he has a working relationship with a number of state senators. . . . He’s a known quantity,” Robin said. “I also felt that he would not necessarily put himself in lock step with either party and feel that he absolutely had to vote Republican or Democrat.”

Leesburg resident Kathy Shupe said she and her husband — who generally votes Republican — both voted for Wexton on Tuesday morning at Sugarland Elementary in Loudoun.

“He has deviated from his party since the tea party took over,” Shupe, 55, said of her husband. “For myself, I have a lot of concerns about a woman’s right to choose.”

Shupe said she was drawn to Wexton’s pro-choice platform, as well as her pledge to focus on strengthening Virginia’s economy.

Whitbeck campaign volunteer Maureen Whalen said she was heartened by the number of people who had come through the precinct at Harper Park Middle School in Leesburg by mid-morning.

“Around 9 o’clock, I noticed there were people waiting outside the door,” she said. “The turnout is very good, and I don’t think they were expecting that.”

Whalen, a 69-year-old retired teacher, said she was inspired to volunteer for Whitbeck because of his commitment to lowering taxes and improving transportation.

“We are overtaxed, and I think he’s for responsible government,” she said. “And he wants to bring our transportation dollars home from Richmond.”

Laura Vozzella contributed to this report.