Typically, House of Delegates elections in Virginia are all about everyday concerns — traffic congestion, how to improve schools and where best to use state tax revenue.

But in a year when Republican Gov. Robert F. McDonnell is embroiled in a gifts scandal and the battle to replace him has been ugly and divisive, several key House races in Northern Virginia are more driven by the hunger for leverage in Richmond.

Democrats, who hold just 32 of 100 seats in the House of Delegates, have been targeting swing districts held by Republicans in Fairfax, Loudoun and Prince William counties in the hope of gaining enough victories to make the difference in key votes during the next session.

If Democrats could gain two additional House seats, the party would be able to sustain a gubernatorial veto, said Del. David Toscano (D-Charlottesville), the House Democratic leader. Three additional seats would mean an extra member on key House committees, such as appropriations, education and transportation, he said.

“It’s tremendously important, because for every additional person, there is an additional vote that may swing one way or the other on an important issue, whether it’s reproductive health or funding for education,” Toscano said. “We’re working very hard to put resources behind the candidates to make sure they’ve got a shot.”

One race that has heated up in recent weeks is in the 34th District, a winding strip of pristine single-family homes along the Potomac River that includes Great Falls, Wolf Trap National Park and parts of McLean.

There, incumbent Barbara Comstock (R) is seeking to fend off an aggressive challenge from Democrat Kathleen Murphy, the head of a McLean consulting firm, who worked for the Department of Commerce under President Bill Clinton.

Murphy, 65, has raised almost $280,000 so far, counting former vice president Al Gore as a donor, according to the Virginia Public Access Project. Comstock has raised $617,000, with $40,000 coming from the campaign fund of House Speaker William J. Howell (R-Stafford).

Murphy, whose political career began as a legislative aide to former Texas congressman Charlie Wilson, has sought to characterize Comstock as too conservative for a district that, in 2008 and 2012, slightly favored President Obama.

“I’m running against her record,” Murphy said, citing Comstock’s support for mandating an invasive procedure, called a transvaginal ultrasound, for women contemplating an abortion, as well as her opposition to some gun-control measures.

Comstock, who had several jobs under former president George W. Bush, countered that Murphy is relatively unknown in the district.

Comstock highlighted her work to secure a Capital Beltway sound wall in a portion of McLean and her sponsorship of a 2011 competitive bidding law on transportation contracts that businesses endorsed and labor advocates opposed.

“I have worked across the board and tirelessly on these issues,” Comstock, 54, said. She said that Murphy is too “focused on divisive social issues” and “has no background in the county.”

Republican candidates see voter outreach as a way to counter Democrats’ attempts to take advantage of the scandal surrounding McDonnell, said Garren Shipley, communications director for the Republican Party of Virginia.

Republicans have had a firm grip on the House since 2000. But this year, 30 incumbent seats are being challenged by Democrats and an additional 11 seats are open races, leaving more opportunity for Democratic gains.

“Democrats, this election cycle, are using any weapon at hand,” Shipley said. “The top of the ticket has been very negative and very noisy for a long time. At the House level, you can go in and have that personal connection with voters.”

Republican Del. Thomas Davis Rust of Herndon sought to do that recently while standing in the rain before a parent-teacher meeting at Herndon Elementary School.

“This is where the real work is,” he said, smiling as most parents offered small greetings before they hurried inside.

The former Herndon mayor first won office in 2001 and is a perpetual target of Democrats in the 87th District, which overwhelmingly supported Timothy M. Kaine (D) for U.S. Senate, as well as Obama, in last year’s election.

Rust’s Democrat challenger this year is Jennifer Boysko, a former aide to Fairfax County Supervisor John W. Foust (D-Dranesville), who is emphasizing social justice issues, transportation and a need for better schools.

Boysko, 46, is among dozens of candidates who have been going door-to-door in recent weeks, seeking votes after work or on weekend afternoons with their arms filled with campaign fliers.

Most of them stick to a script of local issues that they know will resonate with voters. Some who are locked in tight contests raise the stakes and tap into larger frustrations with either of the two major parties.

John Bell, a budget consultant for government contractor Booz Allen Hamilton, has campaigned hard against incumbent David Ramadan (R-Loudoun), who won his first election in 2011 by just 30 votes.

Bell has emphasized that Ramadan was subpoenaed in the federal investigation surrounding McDonnell after details emerged that the governor and his family accepted gifts from a prominent Richmond area businessman.

In turn, Ramadan— a jeweler and international trade consultant who has declined to discuss the investigation — accuses Bell of taking “cheap shots.”

Toscano said Democrats are fighting to find their way back to prominence in Richmond.

“You have to claw your way back one seat at a time,” he said.