Barth, who was U.S. Sen. Robert C. Byrd's state director for 21 years until she resigned last week, was prominently mentioned as a possible candidate last year until state Sen. John Unger II, a Berkeley County Democrat, announced his intentions to run last summer.
When it became apparent that Unger wasn't going to file last week, Barth, who said last Saturday that she had planned to keep her options open, stepped up.
Barth is one of three Democratic candidates vying for Capito's seat, along with Thornton Cooper and Richie Robb.
She already has received announcements of support from Democratic U.S. Sens. Byrd and Jay Rockefeller, and Reps. Alan B. Mollohan, D-1st District, and Nick Rahall, D-3rd District.
Barth said she spoke with Unger on Wednesday about entering the race and hoped to unite the party in her bid to be its nominee against Capito, who is unopposed in the May 13 primary.
A minister's daughter, Barth said she has long felt a "moral obligation" to help people.
"It's an honor to seek this office," Barth said.
In a press release about her visit to Martinsburg, Barth credited her work with Byrd for honing "a pretty unique set of political skills."
"The most valuable skill I learned was how to get results for the people of West Virginia in Washington," Barth said. "People in West Virginia are results-oriented. They'll take you at your word, and they'll expect you to back it up. Don't tell a West Virginian you are going to do something if you can't get it done."
Barth pointed out that through Byrd's offices in Charleston and Martinsburg, more than 1,000 West Virginians received help last year.
"I know how to serve the people of the 2nd District because I see them and talk to them every day," she said. "I take their concerns personally because they are my concerns, too. The dollar is shrinking, and people are worried about their mortgages and their health care. The deficit is climbing through the stratosphere, and so is the cost of gas and groceries.
"West Virginians are demanding some fiscal sanity in Washington."