Whoever wins the general election will make history: West Virginia has never elected a female senator.
Their victories also set up a Rust Belt race that will be a battleground for both parties, testing Democrats' ability to make a convincing case against Republicans in Congress and whether Republicans can elect a senator in the state for the first time since 1956.
"Mitt Romney won West Virginia by nearly 27 points, so Tennant has a lot of work to do," said Jennifer Duffy, a campaign analyst at the nonpartisan Cook Political Report. "But Democrats do have the edge in voter registration."
Capito, a daughter of former West Virginia governor Arch Moore, hopes her family's deep roots and her distance from the Republican Party's tea party wing can win over the conservative Democrats who populate the state's Appalachian region.
"This is a small state where it's about relationships," Capito said in an interview on Tuesday. "I'm known as a Republican who can get things done -- who sees problems and solves them, working across the aisle."
Capito has said if elected, she would like to emulate Sen. Lisa Murkowski of Alaska, a Republican who has clashed with the tea party and is a dealmaker on Capitol Hill.
When Capito first launched her campaign, several conservative groups attacked her moderate fiscal record in Congress. The Club for Growth published a memo decrying the many state projects to which she ladled federal dollars.
But to the relief of national GOP officials, who see Capito as one of the party's best chances of electing another Republican woman to the Senate, a major primary rival never emerged.
Tennant, a youthful former television reporter who has twice won statewide, believes the Democrats' almost 2-to-1 voter-registration edge and the sprawling political machine of the state party -- it controls the governor's office and the state legislature -- leave her well positioned.
Her campaign also argues that Capito will not be able to cast herself as an agreeable centrist, due to her support for the House Republican leadership's economic policies. Tennant adviser Jennifer Donohue said that agenda will not play well in a state full of blue-collar voters, and noted that Tennant has been endorsed by the West Virginia AFL-CIO and other labor unions.
Republicans are planning to vigorously tie Tennant to the Obama administration's regulatory policies. In one Web video, Capito claims President Obama is waging a "war on coal."
Sensing her potential vulnerability on energy, Tennant on the trail has pledged to "stand up" to the Obama administration and other "Washington Democrats." And she started early, knocking the president during her campaign announcement tour last year and talking up her outsider credentials.
Tennant previously lost in a 2011 Democratic gubernatorial primary. The state's current governor, Democrat Earl Ray Tomblin, has endorsed her, as has West Virginia's other senator, Joe Manchin (D).
Democrats currently have a 55-seat majority in the Senate, meaning Republicans need to win a net of six seats in November in order to take over the chamber. Polling in West Virginia has been sporadic; one February survey by Rasmussen Reports shows Capito with a double-digit lead.
During her victory speech on Tuesday, Tennant turned swiftly toward the general election and challenged Capito to five televised debates.
“I am proud of my record and I am proud of my vision for West Virginia," she said. "I will stand up and defend both anywhere, anytime."
Retiring Sen. Jay Rockefeller (D), 76, currently occupies the seat. He was first elected in 1984 after serving as governor and is the great-grandson of John D. Rockefeller, a founder of Standard Oil.