This is part of a series on the Aug. 23 General Assembly primary elections in Northern Virginia. The series will appear in August.

Winning the Democratic primary for Virginia’s blue-leaning 30th Senate District is up to the candidates. And their legs. And their knuckles.

There’s an off-year primary during the peak of vacation season — Aug. 23 — and there are virtually no policy differences among the three Democratic candidates. So the one who best woos voters by walking the district and knocking on the most doors will have a strong shot at replacing the soon-to-retire Patricia S. Ticer (D-Alexandria).

“What really matters to voters in a case like this . . . is just the chance to meet these people door- to-door,” said Stephen Farnsworth, an expert on Virginia politics and an associate professor at George Mason University. “These races tend to go to those who work the hardest.”

And the Democratic candidates — Alexandria City Council member Rob Krupicka, Del. Adam P. Ebbin (Alexandria) and Arlington County School Board member Libby T. Garvey — are working hard. Republican candidate Tim McGhee, a church administrator, is running uncontested.

“In these last two weeks, it’s about reinforcing our message,” Krupicka said. “It’s about going back to the thousands and thousands of people we’ve talked to and checking back with them one more time, and telling them where and how to go vote. This is a very grass-roots campaign, so it’s going to end in a very grass-roots way.”

Krupicka touts his grass-roots experience as well as his work on the state Board of Education and the nonpartisan Pew Charitable Trusts. Ebbin highlights his tenure in the General Assembly. “My experience will allow me to move across the hall to the Senate and be effective on Day One,” he said. (He is giving up his House seat to run for the Senate.)

Garvey said her background as a mother, widow and breast cancer survivor, as well as her work with the Council of Governments, makes her a good replacement for Ticer. If elected, Garvey would follow a long line of female Democratic legislators from Northern Virginia. “I am absolutely confident I’m meant to be in this race. I believe I should win this race,” she said. “I am pedal to the metal. I am working flat-out.”

In an interview, McGhee, the Republican, said he would work to uphold the U.S. and Virginia constitutions, as well as the general principles of life, liberty and justice. Although he hesitated to prioritize any single issue, he said he wanted to work on transportation issues and serve as a voice for those who find problems with the new federal health-care law.

“By sticking to the fundamentals as I am and intend to do . . . it is a message that resonates with not only voters but people who [generally] want a good legislator,” he said.

McGhee is facing an uphill battle. Redistricting this year led to slightly more Republicans in the district, but experts say the move would probably not prevent a Democratic win.

Money is another factor, observers said. The Democratic candidates have raised more than $170,000 each, according to the Virginia Public Access Project. McGhee has about $3,000 in the coffers. (He says he expects the fundraising to “improve greatly.”)

Experts say the three Democrats have raised enough money to run comparable campaigns. On the issues, they are more or less similar: They prioritize transportation, sustainability, the economy and education as issues. To varying degrees, they consider tax reform, veterans affairs, civil liberties and health care important, too.

Among the most important issues for constituents is the increased traffic expected on Interstate 395 as federal employees move to the Mark Center in Alexandria as part of base realignment and closure moves by the Pentagon. All three have suggested ramping up public transportation in the area.

The trio has also chosen the same state villain: Ken T. Cuccinelli II, Virginia’s attorney general and a stalwart conservative. In his first 100 days in office, Cuccinelli sued to block the federal health-care law on constitutional grounds and advised public colleges and universities that they lacked authority to ban discrimination on the basis of sexual orientation, among other things. (Although they say they would work with Republicans on bipartisan legislation, the Democrats disagree with Cuccinelli’s positions.)

“It’s so difficult for voters to figure out what the real difference is between these people,” said Craig Brians, a political scientist and associate professor at Virginia Tech. “Can you tell I’m not really a fan of primaries?”

Although experts call this a hotly contested primary race, it’s still a primary contest, and historically such events have generated low turnouts. Moreover, state redistricting has pushed the primary from June to August, when voters are often on vacation. Farnsworth estimates that 6 to 7 percent of the electorate will vote.

As they canvassed in the district — which represents parts of Alexandria and Arlington and Fairfax counties — the candidates received mixed messages from potential voters.

“We’ve been hit hot and heavy by all the candidates,” said Barbara Kerner, 66, standing outside her Alexandria house. “It’s a bit overkill.”

Others were a bit more receptive. Ebbin found a supporter, Michele K. House, who recognized him as he knocked on the door.

“It’s so good that you came out in this heat,” said House, 60, standing outside her side door in Arlington. “Who’s your opponent? Your fiercer opponent?”

Slightly taken aback, Ebbin began, “I think any of the three of us” — he paused — “but I would say Rob.”

“I think I will vote for you,” House said. “Libby hasn’t come over here, and I like when people go door-to-door.”

But Garvey visited the house of Julianne J. Cronin, 71, who would have disagreed with House. “You have had quite a life,” Cronin, of Alexandria, told Garvey. “I have thought very highly of you. You certainly are right there for us.”

“Can you put up a sign?” Garvey asked. Cronin nodded. “Oh, bless you,” Garvey said. “Can I hug you?”

The next 12 days are “when one candidate has the chance to seal the deal with the voters,” said Mark Rozell, a public policy professor at George Mason University. “This is the critical phase of the campaign.”